New UMA stats for stacking container opt-in measurements.

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index ad7140e7d9eec108e4dfdb6e05ef0952b3217262..c3962750e71970456864ac2b6e4235ecea00f8c0 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -133,7 +133,7 @@ RenderLayer::RenderLayer(RenderLayerModelObject* renderer)
     , m_hasOutOfFlowPositionedDescendantDirty(true)
     , m_hasUnclippedDescendant(false)
     , m_isUnclippedDescendant(false)
-    , m_needsCompositedScrolling(false)
+    , m_isScrollableAreaHasBeenRecorded(false)
     , m_canBePromotedToStackingContainer(false)
     , m_canBePromotedToStackingContainerDirty(true)
     , m_isRootLayer(renderer->isRenderView())
@@ -154,6 +154,7 @@ RenderLayer::RenderLayer(RenderLayerModelObject* renderer)
 #endif
     , m_canSkipRepaintRectsUpdateOnScroll(renderer->isTableCell())
     , m_hasFilterInfo(false)
+    , m_needsCompositedScrolling(MixedTriState)
     , m_blendMode(BlendModeNormal)
     , m_renderer(renderer)
     , m_parent(0)
@@ -2051,7 +2052,7 @@ bool RenderLayer::needsToBeStackingContainer() const
 {
     switch (m_forceNeedsCompositedScrolling) {
     case DoNotForceCompositedScrolling:
-        return m_needsCompositedScrolling;
+        return m_needsCompositedScrolling == TrueTriState;
     case CompositedScrollingAlwaysOn:
         return true;
     case CompositedScrollingAlwaysOff:
@@ -2059,7 +2060,7 @@ bool RenderLayer::needsToBeStackingContainer() const
     }
 
     ASSERT_NOT_REACHED();
-    return m_needsCompositedScrolling;
+    return m_needsCompositedScrolling == TrueTriState;
 }
 
 void RenderLayer::updateNeedsCompositedScrolling()
@@ -2086,10 +2087,19 @@ void RenderLayer::updateNeedsCompositedScrolling()
 
 void RenderLayer::setNeedsCompositedScrolling(bool needsCompositedScrolling)
 {
-    if (m_needsCompositedScrolling == needsCompositedScrolling)
+    const bool neededCompositedScrolling = m_needsCompositedScrolling == TrueTriState;
+    if (neededCompositedScrolling == needsCompositedScrolling)
         return;
 
-    m_needsCompositedScrolling = needsCompositedScrolling;
+    // Count the total number of RenderLayers which need composited scrolling at
+    // some point. This should be recorded at most once per RenderLayer, so we
+    // check if m_needsCompositedScrolling has been set yet. If it hasn't been
+    // set, its value will be MixedTriState, but if it has been set, it will be
+    // TrueTriState or FalseTriState.
+    if (acceleratedCompositingForOverflowScrollEnabled() && m_needsCompositedScrolling == MixedTriState)
+        HistogramSupport::histogramEnumeration("Renderer.NeedsToBeStackingContainer", 1, 2);
+
+    m_needsCompositedScrolling = needsCompositedScrolling ? TrueTriState : FalseTriState;
 
     // Note, the z-order lists may need to be rebuilt, but our code guarantees
     // that we have not affected stacking, so we will not dirty
@@ -6187,6 +6197,13 @@ void RenderLayer::updateScrollableAreaSet(bool hasOverflow)
     if (hasOverflow && isVisibleToHitTest) {
         if (frameView->addScrollableArea(scrollableArea()))
             compositor()->setNeedsUpdateCompositingRequirementsState();
+
+        // Count the total number of RenderLayers that are scrollable areas for
+        // any period. We only want to record this at most once per RenderLayer.
+        if (!m_isScrollableAreaHasBeenRecorded) {
+            HistogramSupport::histogramEnumeration("Renderer.IsScrollableArea", 1, 2);
+            m_isScrollableAreaHasBeenRecorded = true;
+        }

[Ian Vollick, 2013/09/03 18:28:07]

This should happen inside the previous 'if', I think. if (frameView->add...) {
compositor()->...; <new code>; }

[hartmanng, 2013/09/03 21:26:56]

Done.

     } else {
         if (frameView->removeScrollableArea(scrollableArea()))
             setNeedsCompositedScrolling(false);