Real instance methods for BrowserContext: remaining 5 methods.

This CL refactors non-overridable (i.e. implemented only within
//content) methods of `BrowserContext`, so that they are implemented as
instance methods (rather than as `static` methods taking a `self`
argument).  This particular CL refactors the following methods:
- GetMountPoints
- GetSharedCorsOriginAccessList
- EnsureResourceContextInitialized
- NotifyWillBeDestroyed
- SaveSessionState

For the rationale/motivation for this change, please see the CL
description of an earlier CL in this series - r877482.

Bug: 1179776
Change-Id: Ia72acf439e857685592193618baa6d4167983c9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2892048
Commit-Queue: Łukasz Anforowicz <[email protected]>
Commit-Queue: Charlie Reis <[email protected]>
Auto-Submit: Łukasz Anforowicz <[email protected]>
Reviewed-by: Charlie Reis <[email protected]>
Reviewed-by: Trent Apted <[email protected]>
Owners-Override: Charlie Reis <[email protected]>
Cr-Commit-Position: refs/heads/master@{#884783}
diff --git a/chrome/browser/platform_util_unittest.cc b/chrome/browser/platform_util_unittest.cc
index 5212e4f..4456b23 100644
--- a/chrome/browser/platform_util_unittest.cc
+++ b/chrome/browser/platform_util_unittest.cc
@@ -54,7 +54,7 @@
       std::vector<std::unique_ptr<storage::FileSystemBackend>>*
           additional_backends) override {
     storage::ExternalMountPoints* external_mount_points =
-        content::BrowserContext::GetMountPoints(browser_context);
+        browser_context->GetMountPoints();
 
     // New FileSystemBackend that uses our MockSpecialStoragePolicy.
     additional_backends->push_back(
@@ -81,9 +81,9 @@
         content::SetBrowserClientForTesting(content_browser_client_.get());
 
     // The test_directory needs to be mounted for it to be accessible.
-    content::BrowserContext::GetMountPoints(GetProfile())
-        ->RegisterFileSystem("test", storage::kFileSystemTypeLocal,
-                             storage::FileSystemMountOption(), test_directory);
+    GetProfile()->GetMountPoints()->RegisterFileSystem(
+        "test", storage::kFileSystemTypeLocal, storage::FileSystemMountOption(),
+        test_directory);
 
     // To test opening a file, we are going to register a mock extension that
     // handles .txt files. The extension doesn't actually need to exist due to