cc: Make PrepareToDraw return an enum for why it aborts

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index 2b85e321c33940f88a560c34e23403035e36d59d..c8619b9b7fc6e5076f4f3d29d3889006e38f31f9 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -30,6 +30,7 @@
 #include "cc/quads/render_pass.h"
 #include "cc/resources/resource_provider.h"
 #include "cc/resources/tile_manager.h"
+#include "cc/scheduler/draw_swap_readback_result.h"
 #include "skia/ext/refptr.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/gfx/rect.h"
@@ -179,8 +180,9 @@ class CC_EXPORT LayerTreeHostImpl
   // to avoid displaying the frame. If PrepareToDraw is called, DidDrawAllLayers
   // must also be called, regardless of whether DrawLayers is called between the
   // two.
-  virtual bool PrepareToDraw(FrameData* frame,
-                             const gfx::Rect& damage_rect);
+  virtual DrawSwapReadbackResult::DrawResult PrepareToDraw(
+      FrameData* frame,
+      const gfx::Rect& damage_rect);
   virtual void DrawLayers(FrameData* frame, base::TimeTicks frame_begin_time);
   // Must be called if and only if PrepareToDraw was called.
   void DidDrawAllLayers(const FrameData& frame);
@@ -481,10 +483,10 @@ class CC_EXPORT LayerTreeHostImpl
 
   void UpdateTileManagerMemoryPolicy(const ManagedMemoryPolicy& policy);
 
-  // Returns false if the frame should not be displayed. This function should
-  // only be called from PrepareToDraw, as DidDrawAllLayers must be called
-  // if this helper function is called.
-  bool CalculateRenderPasses(FrameData* frame);
+  // This function should only be called from PrepareToDraw, as DidDrawAllLayers
+  // must be called if this helper function is called.  Returns DID_DRAW if
+  // the frame should be drawn.
+  DrawSwapReadbackResult::DrawResult CalculateRenderPasses(FrameData* frame);
 
   void SendReleaseResourcesRecursive(LayerImpl* current);
   bool EnsureRenderSurfaceLayerList();