Added a UMA metric to track slow path scrolling.

cc/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layer_tree_host_impl.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
 #include "base/json/json_writer.h"
+#include "base/metrics/histogram.h"
 #include "base/stl_util.h"
 #include "cc/append_quads_data.h"
 #include "cc/compositor_frame_metadata.h"
 #include "cc/damage_tracker.h"
 #include "cc/debug_rect_history.h"
 #include "cc/delay_based_time_source.h"
 #include "cc/delegating_renderer.h"
 #include "cc/frame_rate_counter.h"
 #include "cc/gl_renderer.h"
 #include "cc/heads_up_display_layer_impl.h"
@@ skipping 470 matching lines @@
                 appendQuadsData.hadOcclusionFromOutsideTargetSurface |= hasOcclusionFromOutsideTargetSurface;
             else {
                 DCHECK_EQ(activeTree(), it->layerTreeImpl());
                 it->willDraw(m_resourceProvider.get());
                 frame.willDrawLayers.push_back(*it);
 
                 if (it->hasContributingDelegatedRenderPasses()) {
                     RenderPass::Id contributingRenderPassId = it->firstContributingRenderPassId();
                     while (frame.renderPassesById.find(contributingRenderPassId) != frame.renderPassesById.end()) {
                         RenderPass* renderPass = frame.renderPassesById[contributingRenderPassId];
-  
+
                         AppendQuadsData appendQuadsData(renderPass->id);
                         appendQuadsForLayer(renderPass, *it, occlusionTracker, appendQuadsData);
 
                         contributingRenderPassId = it->nextContributingRenderPassId(contributingRenderPassId);
                     }
                 }
 
                 appendQuadsForLayer(targetRenderPass, *it, occlusionTracker, appendQuadsData);
             }
 
@@ skipping 641 matching lines @@
     // First find out which layer was hit from the saved list of visible layers
     // in the most recent frame.
     LayerImpl* layerImpl = LayerTreeHostCommon::findLayerThatIsHitByPoint(deviceViewportPoint, activeTree()->RenderSurfaceLayerList());
 
     // Walk up the hierarchy and look for a scrollable layer.
     LayerImpl* potentiallyScrollingLayerImpl = 0;
     for (; layerImpl; layerImpl = layerImpl->parent()) {
         // The content layer can also block attempts to scroll outside the main thread.
         if (layerImpl->tryScroll(deviceViewportPoint, type) == ScrollOnMainThread) {
             m_numMainThreadScrolls++;
+            UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
             return ScrollOnMainThread;
         }
 
         LayerImpl* scrollLayerImpl = findScrollLayerForContentLayer(layerImpl);
         if (!scrollLayerImpl)
             continue;
 
         ScrollStatus status = scrollLayerImpl->tryScroll(deviceViewportPoint, type);
 
         // If any layer wants to divert the scroll event to the main thread, abort.
         if (status == ScrollOnMainThread) {
             m_numMainThreadScrolls++;
+            UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
             return ScrollOnMainThread;
         }
 
+        UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", false);

[aelias_OOO_until_Jul13, 2013/01/29 22:00:08]

Please put this one right before ScrollStarted since here it would trigger
multiple times if the scroll gets bubbled up through the layer tree.

[clholgat, 2013/01/31 18:55:50]

Done.

         if (status == ScrollStarted && !potentiallyScrollingLayerImpl)
             potentiallyScrollingLayerImpl = scrollLayerImpl;
     }
 
     if (potentiallyScrollingLayerImpl) {
         m_activeTree->set_currently_scrolling_layer(potentiallyScrollingLayerImpl);
         // Gesture events need to be transformed from viewport coordinates to local layer coordinates
         // so that the scrolling contents exactly follow the user's finger. In contrast, wheel
         // events are already in local layer coordinates so we can just apply them directly.
         m_scrollDeltaIsInViewportSpace = (type == Gesture);
@@ skipping 504 matching lines @@
     LayerImpl* layer = getNonCompositedContentLayerRecursive(tree->RootLayer());
     return layer ? layer->getPicture() : skia::RefPtr<SkPicture>();
 }
 
 void LayerTreeHostImpl::savePaintTime(const base::TimeDelta& totalPaintTime)
 {
     m_paintTimeCounter->SavePaintTime(totalPaintTime);
 }
 
 }  // namespace cc