cc: Add a checkerboard trace event instant to picture layer impl.

This is a small patch that adds a trace event instant whenever we draw
checkerboards. This is useful for debugging or figuring out exactly
when we checkerboarded.

R=enne

Review URL: https://codereview.chromium.org/275173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269631 0039d316-1c4b-4281-b951-d872f2087c98

cc/layers/picture_layer_impl.cc

@@ -234,6 +234,7 @@ void PictureLayerImpl::AppendQuads(QuadSink* quad_sink,
   // unused can be considered for removal.
   std::vector<PictureLayerTiling*> seen_tilings;
 
+  bool had_checkerboard_quads = false;
   for (PictureLayerTilingSet::CoverageIterator iter(
       tilings_.get(), contents_scale_x(), rect, ideal_contents_scale_);
        iter;
@@ -248,6 +249,7 @@ void PictureLayerImpl::AppendQuads(QuadSink* quad_sink,
         visible_geometry_rect.width() * visible_geometry_rect.height();
 
     if (!*iter || !iter->IsReadyToDraw()) {
+      had_checkerboard_quads = true;
       if (draw_checkerboard_for_missing_tiles()) {
         scoped_ptr<CheckerboardDrawQuad> quad = CheckerboardDrawQuad::Create();
         SkColor color = DebugColors::DefaultCheckerboardColor();
@@ -343,6 +345,12 @@ void PictureLayerImpl::AppendQuads(QuadSink* quad_sink,
       seen_tilings.push_back(iter.CurrentTiling());
   }
 
+  if (had_checkerboard_quads) {
+    TRACE_EVENT_INSTANT0("cc",
+                         "PictureLayerImpl::AppendQuads checkerboard",
+                         TRACE_EVENT_SCOPE_THREAD);
+  }
+
   // Aggressively remove any tilings that are not seen to save memory. Note
   // that this is at the expense of doing cause more frequent re-painting. A
   // better scheme would be to maintain a tighter visible_content_rect for the