Update web-platform-tests URLs after repo move

Part of https://bit.ly/wpt-transition.

Most of the changes were done as such:
```
git grep -l w3c/web-platform-tests | xargs sed -i 's#https://github.com/w3c/web-platform-tests#https://github.com/web-platform-tests/wpt#g'
git grep -lF 'web-platform-tests.org' | xargs sed -i 's#http://web-platform-tests.org#https://web-platform-tests.org#g'
```

The http => https changes included here because one such URL got
involved in the first after manual tweaks.

Change-Id: Ied8334b88e4a0936cd995a002bb2a502c04ecfc7
Reviewed-on: https://chromium-review.googlesource.com/1080510
Reviewed-by: Tim Volodine <[email protected]>
Reviewed-by: Robert Ma <[email protected]>
Reviewed-by: Rick Byers <[email protected]>
Commit-Queue: Philip Jägenstedt <[email protected]>
Cr-Commit-Position: refs/heads/master@{#563242}
diff --git a/docs/testing/writing_layout_tests.md b/docs/testing/writing_layout_tests.md
index 7d90642..f71eb3a 100644
--- a/docs/testing/writing_layout_tests.md
+++ b/docs/testing/writing_layout_tests.md
@@ -73,7 +73,7 @@
 
 Tests should be written under the assumption that they will be upstreamed
 to the WPT project. To this end, tests should follow the
-[WPT guidelines](http://web-platform-tests.org/writing-tests/).
+[WPT guidelines](https://web-platform-tests.org/writing-tests/).
 
 
 There is no style guide that applies to all layout tests. However, some projects
@@ -93,18 +93,18 @@
 less reliable tests.
 
 All new JavaScript tests should be written using the
-[testharness.js](https://github.com/w3c/web-platform-tests/tree/master/resources)
+[testharness.js](https://github.com/web-platform-tests/wpt/tree/master/resources)
 testing framework. This framework is used by the tests in the
-[web-platform-tests](https://github.com/w3c/web-platform-tests) repository,
+[web-platform-tests](https://github.com/web-platform-tests/wpt) repository,
 which is shared with all the other browser vendors, so `testharness.js` tests
 are more accessible to browser developers.
 
-See the [API documentation](http://web-platform-tests.org/writing-tests/testharness-api.html)
+See the [API documentation](https://web-platform-tests.org/writing-tests/testharness-api.html)
 for a thorough introduction to `testharness.js`.
 
 Layout tests should follow the recommendations of the above documentation.
 Furthermore, layout tests should include relevant
-[metadata](http://web-platform-tests.org/writing-tests/css-metadata.html). The
+[metadata](https://web-platform-tests.org/writing-tests/css-metadata.html). The
 specification URL (in `<link rel="help">`) is almost always relevant, and is
 incredibly helpful to a developer who needs to understand the test quickly.
 
@@ -190,7 +190,7 @@
 This is contrary to the WPT guidelines, which call for absolute paths.
 This limitation does not apply to the tests in `LayoutTests/http`, which rely on
 an HTTP server, or to the tests in `LayoutTests/external/wpt`, which are
-imported from the [WPT repository](https://github.com/w3c/web-platform-tests).
+imported from the [WPT repository](https://github.com/web-platform-tests/wpt).
 ***
 
 ### WPT Supplemental Testing APIs
@@ -362,7 +362,7 @@
 cannot be covered by JavaScript tests.
 
 New reference tests should follow the
-[WPT reftests guidelines](http://web-platform-tests.org/writing-tests/reftests.html).
+[WPT reftests guidelines](https://web-platform-tests.org/writing-tests/reftests.html).
 The most important points are summarized below.
 
 * &#x1F6A7; The test page declares the reference page using a
@@ -435,7 +435,7 @@
 Pixel tests should still follow the principles laid out above. Pixel tests pose
 unique challenges to the desire to have *self-describing* and *cross-platform*
 tests. The
-[WPT rendering test guidelines](http://web-platform-tests.org/writing-tests/rendering.html)
+[WPT rendering test guidelines](https://web-platform-tests.org/writing-tests/rendering.html)
 contain useful guidance. The most relevant pieces of advice are below.
 
 * Whenever possible, use a green paragraph / page / square to indicate success.