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 42aa9533c928df6b108bf063cb76bb91c546d130..85ce7382d7ab79771109a6562a9af764daec0723 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"
@@ -479,7 +480,7 @@ 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);
 
@@ -1219,6 +1220,7 @@ 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;
         }
 
@@ -1231,6 +1233,7 @@ 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;
         }
 
@@ -1246,6 +1249,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
         m_scrollDeltaIsInViewportSpace = (type == Gesture);
         m_numImplThreadScrolls++;
         m_client->renewTreePriority();
+        UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", false);
         return ScrollStarted;
     }
     return ScrollIgnored;