[chromecast] Use null SkSurface instead of bitmap-backed one.

On audio-only devices there is no need to perform rasterization for
real. We just need a dummy SkSurface that simply ignores all draw calls.
Using the null surface saves 3.6M memory and draw calls are much faster.

Bug: 737726
Change-Id: Ic11ee7afc80dd71a6ae5d5d2ba5e4aae513fcb61
Reviewed-on: https://chromium-review.googlesource.com/594643Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Alok Priyadarshi <alokp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490794}

ui/ozone/platform/cast/surface_factory_cast.cc

@@ -29,8 +29,8 @@ class DummySurface : public SurfaceOzoneCanvas {
   sk_sp<SkSurface> GetSurface() override { return surface_; }
 
   void ResizeCanvas(const gfx::Size& viewport_size) override {
-    surface_ = SkSurface::MakeRaster(SkImageInfo::MakeN32Premul(
-        viewport_size.width(), viewport_size.height()));
+    surface_ =
+        SkSurface::MakeNull(viewport_size.width(), viewport_size.height());
   }
 
   void PresentCanvas(const gfx::Rect& damage) override {}