Record histogram when succeeding in creating a DC layer overlay.

This helps in creating a baseline for comparing other numbers against.
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2908623006
Cr-Commit-Position: refs/heads/master@{#475747}

cc/output/dc_layer_overlay.cc

@@ -205,6 +205,7 @@ void DCLayerOverlayProcessor::Process(ResourceProvider* resource_provider,
     }
     overlay_damage_rect->Union(quad_rectangle);
 
+    RecordDCLayerResult(DC_LAYER_SUCCESS);
     ca_layer_overlays->push_back(ca_layer);
     // Only allow one overlay for now.
     break;