[CI] Make FrameIsScrollableDidChange return false when there's no change

FrameIsScrollableDidChange had a typo and always returned true. This
prevented an early-out in ScrollingCoordinator's
UpdateAfterCompositingChangeIfNeeded from being effective.

This patch should have no web-visible changes and should only improve
performance by making an early-out work.

Bug: 826883
Change-Id: Ic3bbac06b6cf7e6a1097a42ba43d1dfd35dcb0c6
Reviewed-on: https://chromium-review.googlesource.com/994546Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548101}

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

@@ -4051,7 +4051,7 @@ void LocalFrameView::RemoveResizerArea(LayoutBox& resizer_box) {
 
 bool LocalFrameView::FrameIsScrollableDidChange() {
   DCHECK(GetFrame().IsLocalRoot());
-  return GetScrollingContext()->WasScrollable() ==
+  return GetScrollingContext()->WasScrollable() !=
          LayoutViewportScrollableArea()->IsScrollable();
 }
 

third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinatorTest.cpp

@@ -1132,6 +1132,37 @@ TEST_P(ScrollingCoordinatorTest, StickyTriggersMainThreadScroll) {
             scroll_layer->MainThreadScrollingReasons());
 }
 
+// LocalFrameView::FrameIsScrollableDidChange is used as a dirty bit and is
+// set to clean in ScrollingCoordinator::UpdateAfterCompositingChangeIfNeeded.
+// This test ensures that the dirty bit is set and unset properly.
+TEST_P(ScrollingCoordinatorTest, FrameIsScrollableDidChange) {
+  LoadHTML(R"HTML(
+    <div id='bg' style='background: red; width: 10px; height: 10px;'></div>
+    <div id='forcescroll' style='height: 5000px;'></div>
+  )HTML");
+
+  // Initially there is a change but that goes away after a compositing update.
+  EXPECT_TRUE(GetFrame()->View()->FrameIsScrollableDidChange());
+  ForceFullCompositingUpdate();
+  EXPECT_FALSE(GetFrame()->View()->FrameIsScrollableDidChange());
+
+  // A change to background color should not change the frame's scrollability.
+  auto* background = GetFrame()->GetDocument()->getElementById("bg");
+  background->removeAttribute(HTMLNames::styleAttr);
+  EXPECT_FALSE(GetFrame()->View()->FrameIsScrollableDidChange());
+
+  ForceFullCompositingUpdate();
+
+  // Making the frame not scroll should change the frame's scrollability.
+  auto* forcescroll = GetFrame()->GetDocument()->getElementById("forcescroll");
+  forcescroll->removeAttribute(HTMLNames::styleAttr);
+  GetFrame()->View()->UpdateLifecycleToLayoutClean();
+  EXPECT_TRUE(GetFrame()->View()->FrameIsScrollableDidChange());
+
+  ForceFullCompositingUpdate();
+  EXPECT_FALSE(GetFrame()->View()->FrameIsScrollableDidChange());
+}
+
 class NonCompositedMainThreadScrollingReasonTest
     : public ScrollingCoordinatorTest {
   static const uint32_t kLCDTextRelatedReasons =