Added a UMA metric to track slow path scrolling.

cc/layer_tree_host_impl.cc

Index: cc/layer_tree_host_impl.cc
diff --git a/cc/layer_tree_host_impl.cc b/cc/layer_tree_host_impl.cc
index 5f186527744e2ab694741efd27c326687012e053..a65fbe638fcd60ddc7f8c0ccfa48aa16a9dde014 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -9,6 +9,7 @@
 #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"
@@ -499,7 +500,7 @@ bool LayerTreeHostImpl::calculateRenderPasses(FrameData& frame)
                     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);
 
@@ -1161,6 +1162,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
         // 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;
         }
 
@@ -1173,9 +1175,11 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
         // 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;
     }