CanvasResourceDispatcher: add interesting TRACE_EVENT0()

This CL adds TRACE_EVENT0() to some important CanvasResourceDispatcher
methods: DispatchFrame{Sync}() and PrepareFrame(), which are now used
more frequently, for lowLatency cases.

Bug: 869161
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I66dfd2f47eaba0183d34c5e1f09f759d6f0fcf1b
Reviewed-on: https://chromium-review.googlesource.com/1161974Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580634}

third_party/blink/renderer/platform/graphics/canvas_resource_dispatcher.cc

@@ -131,6 +131,7 @@ void CanvasResourceDispatcher::DispatchFrameSync(
     base::TimeTicks commit_start_time,
     const SkIRect& damage_rect,
     bool needs_vertical_flip) {
+  TRACE_EVENT0("blink", "CanvasResourceDispatcher::DispatchFrameSync");
   viz::CompositorFrame frame;
   if (!PrepareFrame(std::move(canvas_resource), commit_start_time, damage_rect,
                     needs_vertical_flip, &frame)) {
@@ -150,6 +151,7 @@ void CanvasResourceDispatcher::DispatchFrame(
     base::TimeTicks commit_start_time,
     const SkIRect& damage_rect,
     bool needs_vertical_flip) {
+  TRACE_EVENT0("blink", "CanvasResourceDispatcher::DispatchFrame");
   viz::CompositorFrame frame;
   if (!PrepareFrame(std::move(canvas_resource), commit_start_time, damage_rect,
                     needs_vertical_flip, &frame)) {
@@ -168,6 +170,7 @@ bool CanvasResourceDispatcher::PrepareFrame(
     const SkIRect& damage_rect,
     bool needs_vertical_flip,
     viz::CompositorFrame* frame) {
+  TRACE_EVENT0("blink", "CanvasResourceDispatcher::PrepareFrame");
   if (!canvas_resource || !VerifyImageSize(canvas_resource->Size()))
     return false;