cc: Add traces for identification of raster buffer.

This patch adds traces in Playback() function depicting which raster
buffer provider is used.

BUG=599863
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2045743003
Cr-Commit-Position: refs/heads/master@{#398477}

diff --git a/cc/raster/zero_copy_raster_buffer_provider.cc b/cc/raster/zero_copy_raster_buffer_provider.cc
index 93e8d7e..b89b45b 100644
--- a/cc/raster/zero_copy_raster_buffer_provider.cc
+++ b/cc/raster/zero_copy_raster_buffer_provider.cc

@@ -36,6 +36,7 @@
       uint64_t new_content_id,
       float scale,
       const RasterSource::PlaybackSettings& playback_settings) override {
+    TRACE_EVENT0("cc", "ZeroCopyRasterBuffer::Playback");
     gfx::GpuMemoryBuffer* buffer = lock_.GetGpuMemoryBuffer();
     if (!buffer)
       return;