Fix CSS clips cannot be correctly handled by composited scroller

BUG=645957
TEST=PaintLayerScrollableAreaTest.OnlyAutoClippedScrollingContentsLayerPromoted
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2595593002
Cr-Commit-Position: refs/heads/master@{#440925}

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

@@ -1762,7 +1762,8 @@ bool PaintLayerScrollableArea::computeNeedsCompositedScrolling(
   // with the results.
   return !(layer->size().isEmpty() || layer->hasDescendantWithClipPath() ||
            layer->hasAncestorWithClipPath() ||
-           layer->layoutObject()->style()->hasBorderRadius());
+           layer->layoutObject()->style()->hasBorderRadius() ||
+           layer->layoutObject()->hasClip());
 }
 
 void PaintLayerScrollableArea::addStyleRelatedMainThreadScrollingReasons(

third_party/WebKit/Source/core/paint/PaintLayerScrollableAreaTest.cpp

@@ -462,4 +462,40 @@ TEST_F(PaintLayerScrollableAreaTest, OverlayScrollbarColorThemeUpdated) {
   ASSERT_EQ(ScrollbarOverlayColorTheme::ScrollbarOverlayColorThemeLight,
             blackLayer->getScrollableArea()->getScrollbarOverlayColorTheme());
 }
+
+// Test that css clip applied to the scroller will cause the
+// scrolling contents layer to not be promoted.
+TEST_F(PaintLayerScrollableAreaTest,
+       OnlyAutoClippedScrollingContentsLayerPromoted) {
+  setBodyInnerHTML(
+      "<style>"
+      ".clip { clip: rect(0px,60px,50px,0px); }"
+      "#scroller { position: absolute; overflow: auto;"
+      "height: 100px; width: 100px; background: grey;"
+      "will-change:transform; }"
+      "#scrolled { height: 300px; }"
+      "</style>"
+      "<div id=\"scroller\"><div id=\"scrolled\"></div></div>");
+  document().view()->updateAllLifecyclePhases();
+
+  Element* scroller = document().getElementById("scroller");
+  PaintLayer* paintLayer =
+      toLayoutBoxModelObject(scroller->layoutObject())->layer();
+  ASSERT_TRUE(paintLayer);
+  EXPECT_TRUE(paintLayer->needsCompositedScrolling());
+
+  // Add clip to scroller.
+  scroller->setAttribute("class", "clip", ASSERT_NO_EXCEPTION);
+  document().view()->updateAllLifecyclePhases();
+  paintLayer = toLayoutBoxModelObject(scroller->layoutObject())->layer();
+  ASSERT_TRUE(paintLayer);
+  EXPECT_FALSE(paintLayer->needsCompositedScrolling());
+
+  // Change the scroller to be auto clipped again.
+  scroller->removeAttribute("class");
+  document().view()->updateAllLifecyclePhases();
+  paintLayer = toLayoutBoxModelObject(scroller->layoutObject())->layer();
+  ASSERT_TRUE(paintLayer);
+  EXPECT_TRUE(paintLayer->needsCompositedScrolling());
+}
 }