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 1a4040519f0786d6a2b5f11c60cf486e43520a89..82758806cc61c69c67e7b7eebf2a89f12c5dc7bf 100644
--- a/cc/resources/tile_manager.cc
+++ b/cc/resources/tile_manager.cc
@@ -11,6 +11,7 @@
 #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/debug/traced_value.h"
 #include "cc/resources/raster_worker_pool.h"
@@ -907,6 +908,11 @@ 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);