Fix rootscroller layout test on slow bots

Add a rAF before starting the test. My guess is that on slow bots
the the gesture starts before the layers are initialized and pushed
to the compositor's acive tree so the entire gesture gets ignored.

Bug: 836623
Change-Id: I1c54e31b4fb064926d8ad94da4bb8d9751332067
Reviewed-on: https://chromium-review.googlesource.com/1034010Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554761}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3217,9 +3217,6 @@ crbug.com/742670 [ Mac ] plugins/iframe-plugin-bgcolor.html [ Failure Pass ]
 
 crbug.com/780398 [ Mac ] plugins/mouse-capture-inside-shadow.html [ Failure Pass ]
 
-# Failing on Trusty Debug, disabled while investigating.
-crbug.com/836623 [ Linux Debug ] virtual/android/rootscroller/gesture-scroll-document-not-root-scroller.html [ Failure ]
-
 crbug.com/678346 [ Debug ] fast/dom/shadow/selections-in-shadow.html [ Pass Timeout ]
 crbug.com/678346 [ Win7 Mac Debug ] storage/indexeddb/index-cursor.html [ Pass Timeout ]
 crbug.com/810254 [ Win7 ] storage/indexeddb/observer-frame.html [ Pass Timeout ]

third_party/WebKit/LayoutTests/rootscroller/gesture-scroll-document-not-root-scroller.html

@@ -65,7 +65,7 @@
     });
   }
 
-  function waitForScrollAndCheck() {
+  function waitForScroll() {
     const MAX_FRAME = 500;
     return new Promise((resolve, reject) => {
       function tick(frames) {
@@ -80,26 +80,28 @@
     });
   }
 
-  window.onload = function() {
+  // Do a rAF before actually starting the test to ensure we leave enough time
+  // to commit layers to the compositor before we start scrolling.
+  window.onload = function() { requestAnimationFrame(() => {
     var rootscroller = document.querySelector('#rootscroller');
     document.rootScroller = rootscroller;
 
     promise_test( t => {
       const GESTURE_SOURCE_TYPE = 2; // MOUSE_INPUT from synthetic_gesture_params.h
       return scrollDown(500, 100, 100, GESTURE_SOURCE_TYPE, 1000)
-          .then(waitForScrollAndCheck)
-          .then(() => {
-            if (window.internals) {
-              assert_equals(window.internals.effectiveRootScroller(document),
-                            rootscroller,
-                            "Failed to set root scroller");
-            }
+        .then(waitForScroll)
+        .then(() => {
+          if (window.internals) {
+            assert_equals(window.internals.effectiveRootScroller(document),
+                          rootscroller,
+                          "Failed to set root scroller");
+          }
 
-            assert_greater_than(
-                document.scrollingElement.scrollTop, 0, "Document must be scrolled");
-            assert_equals(
-                rootscroller.scrollTop, 0, "RootScroller must not be scrolled");
-          });
+          assert_greater_than(
+              document.scrollingElement.scrollTop, 0, "Document must be scrolled");
+          assert_equals(
+              rootscroller.scrollTop, 0, "RootScroller must not be scrolled");
+        });
     }, "Gesture scrolling should scroll document.");
-  }
+  });}
 </script>