Clear the frame to black before drawing

We used to do this in the SkPicture, but it's simpler to do it in the
rasterizer now that we're using multiple SkPictures in Dart.

services/sky/compositor/rasterizer_bitmap.cc

@@ -31,8 +31,7 @@ scoped_ptr<mojo::GLTexture> RasterizerBitmap::Rasterize(SkPicture* picture) {
 
   SkBitmapDevice device(bitmap_);
   SkCanvas canvas(&device);
-  // Draw red so we can see when we fail to paint.
-  canvas.drawColor(SK_ColorRED);
+  canvas.clear(SK_ColorBLACK);
   canvas.drawPicture(picture);
   canvas.flush();
 

services/sky/compositor/rasterizer_ganesh.cc

@@ -29,6 +29,7 @@ scoped_ptr<mojo::GLTexture> RasterizerGanesh::Rasterize(SkPicture* picture) {
                               host_->resource_manager()->CreateTexture(size));
 
   SkCanvas* canvas = surface.canvas();
+  canvas->clear(SK_ColorBLACK);
   canvas->drawPicture(picture);
   canvas->flush();
 

sky/shell/gpu/rasterizer.cc

@@ -66,6 +66,7 @@ void Rasterizer::Draw(PassRefPtr<SkPicture> picture) {
 void Rasterizer::DrawPicture(SkPicture* picture) {
   TRACE_EVENT0("sky", "Rasterizer::DrawPicture");
   SkCanvas* canvas = ganesh_surface_->canvas();
+  canvas->clear(SK_ColorBLACK);
   canvas->drawPicture(picture);
   canvas->flush();
 }