Temporarily enable scroll anchoring on trunk.

This is to get feedback from canary users.  It will be reverted on 2016-09-16.

BUG=624074

Review-Url: https://codereview.chromium.org/2335363003
Cr-Commit-Position: refs/heads/master@{#418715}

third_party/WebKit/LayoutTests/virtual/stable/webexposed/css-properties-as-js-properties-expected.txt

@@ -179,6 +179,7 @@ outlineOffset
 outlineStyle
 outlineWidth
 overflow
+overflowAnchor
 overflowWrap
 overflowX
 overflowY

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

@@ -191,7 +191,8 @@ RootLayerScrolling
 RTCPeerConnectionNewGetStats status=test
 ScriptedSpeech status=stable
 // Scrolls to compensate for layout movements (bit.ly/scroll-anchoring).
-ScrollAnchoring status=experimental, settable_from_internals=True
+// TODO(skobes): Return ScrollAnchoring to experimental status by 9/16.
+ScrollAnchoring status=stable, settable_from_internals=True
 // Implements documentElement.scrollTop/Left and bodyElement.scrollTop/Left
 // as per the spec, matching other Web engines.
 ScrollTopLeftInterop status=experimental