Set removal milestone in MediaStreamTrack.getSources deprecation message.

BUG=388194,649710

Review-Url: https://codereview.chromium.org/2390553003
Cr-Commit-Position: refs/heads/master@{#422762}

third_party/WebKit/LayoutTests/fast/mediastream/MediaStreamTrack-getSources-expected.txt

-CONSOLE WARNING: line 1: MediaStreamTrack.getSources is deprecated. See https://www.chromestatus.com/feature/4765305641369600 for more details.
+CONSOLE WARNING: line 1: MediaStreamTrack.getSources is deprecated and will be removed in M56, around January 2017. See https://www.chromestatus.com/features/4765305641369600 for more details.
 Tests MediaStreamTrack::getSources.
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".

third_party/WebKit/Source/core/frame/Deprecation.cpp

@@ -423,9 +423,8 @@ String Deprecation::deprecationMessage(UseCounter::Feature feature) {
                         "'Performance.onresourcetimingbufferfull'");
 
     case UseCounter::MediaStreamTrackGetSources:
-      return "MediaStreamTrack.getSources is deprecated. See "
-             "https://www.chromestatus.com/feature/4765305641369600 for more "
-             "details.";
+      return willBeRemoved("MediaStreamTrack.getSources", M56,
+                           "4765305641369600");
 
     case UseCounter::WebAnimationHyphenatedProperty:
       return "Hyphenated property names in Web Animations keyframes are "