cc: Add tile-free software compositing mode.

cc/resources/tile_manager.cc

Index: cc/resources/tile_manager.cc
diff --git a/cc/resources/tile_manager.cc b/cc/resources/tile_manager.cc
index 5da623282065a81c0bd4b9c59d28e90fbf6e90fc..9d5b5616531d5146ce98306566f85146ff3c762b 100644
--- a/cc/resources/tile_manager.cc
+++ b/cc/resources/tile_manager.cc
@@ -12,6 +12,7 @@
 #include "base/json/json_writer.h"
 #include "base/logging.h"
 #include "base/metrics/histogram.h"
+#include "cc/debug/debug_colors.h"
 #include "cc/debug/devtools_instrumentation.h"
 #include "cc/resources/raster_worker_pool.h"
 #include "cc/resources/resource_pool.h"
@@ -865,6 +866,11 @@ void TileManager::RunRasterTask(
   SkDevice device(bitmap);
   SkCanvas canvas(&device);
 
+#ifndef NDEBUG
+  // Any non-painted areas will be left in this color.
+  canvas.clear(DebugColors::NonPaintedFillColor());
+#endif  // NDEBUG
+
   if (stats_instrumentation->record_rendering_stats()) {
     PicturePileImpl::RasterStats raster_stats;
     picture_pile->Raster(&canvas, rect, contents_scale, &raster_stats);