Note how to disable the media router in the Mac build instructions

Bug: 1111418
Change-Id: I64d56920ebe10412e311e5ca3b4b633b864e5160
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340794
Auto-Submit: Dominic Mazzoni <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Commit-Queue: Avi Drissman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#795575}
diff --git a/docs/mac_build_instructions.md b/docs/mac_build_instructions.md
index dbbaa7f..4213fd64 100644
--- a/docs/mac_build_instructions.md
+++ b/docs/mac_build_instructions.md
@@ -168,6 +168,14 @@
 $ out/Default/Chromium.app/Contents/MacOS/Chromium
 ```
 
+## Avoiding the "incoming network connections" dialog
+
+Every time you start a new developer build of Chrome you get a system dialog
+asking "Do you want the application Chromium.app to accept incoming
+network connections?" - to avoid this, run with this command-line flag:
+
+--disable-features="MediaRouter"
+
 ## Running test targets
 
 You can run the tests in the same way. You can also limit which tests are