Avoid element id collisions with the visual viewport

The frame scroll layer and the inner viewport layer had previously been
given the same element id.

BUG=658252

Review-Url: https://chromiumcodereview.appspot.com/2443633002
Cr-Commit-Position: refs/heads/master@{#426928}

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

@@ -391,9 +391,10 @@ void VisualViewport::attachToLayerTree(GraphicsLayer* currentLayerTreeRoot) {
         m_innerViewportContainerLayer->platformLayer());
     m_innerViewportScrollLayer->platformLayer()->setUserScrollable(true, true);
     if (mainFrame()) {
-      if (Document* document = mainFrame()->document())
+      if (Document* document = mainFrame()->document()) {
         m_innerViewportScrollLayer->setElementId(createCompositorElementId(
-            DOMNodeIds::idForNode(document), CompositorSubElementId::Scroll));
+            DOMNodeIds::idForNode(document), CompositorSubElementId::Viewport));
+      }
     }
 
     m_rootTransformLayer->addChild(m_innerViewportContainerLayer.get());

third_party/WebKit/Source/platform/graphics/CompositorElementId.h

@@ -10,7 +10,7 @@
 
 namespace blink {
 
-enum class CompositorSubElementId { Primary, Scroll, LinkHighlight };
+enum class CompositorSubElementId { Primary, Scroll, Viewport, LinkHighlight };
 
 using CompositorElementId = cc::ElementId;
 

third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp

@@ -1175,6 +1175,12 @@ void GraphicsLayer::setElementId(const CompositorElementId& id) {
     layer->setElementId(id);
 }
 
+CompositorElementId GraphicsLayer::elementId() const {
+  if (WebLayer* layer = platformLayer())
+    return layer->elementId();
+  return CompositorElementId();
+}
+
 void GraphicsLayer::setCompositorMutableProperties(uint32_t properties) {
   if (WebLayer* layer = platformLayer())
     layer->setCompositorMutableProperties(properties);

third_party/WebKit/Source/platform/graphics/GraphicsLayer.h

@@ -268,6 +268,8 @@ class PLATFORM_EXPORT GraphicsLayer : public WebLayerScrollClient,
   WebLayer* contentsLayer() const { return m_contentsLayer; }
 
   void setElementId(const CompositorElementId&);
+  CompositorElementId elementId() const;
+
   void setCompositorMutableProperties(uint32_t);
 
   ContentLayerDelegate* contentLayerDelegateForTesting() const {

third_party/WebKit/Source/web/tests/VisualViewportTest.cpp

@@ -19,6 +19,7 @@
 #include "platform/PlatformGestureEvent.h"
 #include "platform/geometry/DoublePoint.h"
 #include "platform/geometry/DoubleRect.h"
+#include "platform/graphics/CompositorElementId.h"
 #include "platform/testing/RuntimeEnabledFeaturesTestHelpers.h"
 #include "platform/testing/URLTestHelpers.h"
 #include "public/platform/Platform.h"
@@ -783,6 +784,9 @@ TEST_P(ParameterizedVisualViewportTest,
   // Ensure the scroll layer matches the frame view's size.
   EXPECT_SIZE_EQ(FloatSize(320, 240), visualViewport.scrollLayer()->size());
 
+  EXPECT_EQ(static_cast<int>(CompositorSubElementId::Viewport),
+            visualViewport.scrollLayer()->elementId().secondaryId);
+
   // Ensure the location and scale were reset.
   EXPECT_SIZE_EQ(FloatSize(), visualViewport.scrollOffset());
   EXPECT_EQ(1, visualViewport.scale());