cc: Return a NullCanvas instead of NULL in context lost situations.

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 b96151e5cc037fe5e9370e96ceb8df3606fd56b3..55fb6267a460a0901e9b9e59c8c061298fb9de3e 100644
--- a/cc/resources/tile_manager.cc
+++ b/cc/resources/tile_manager.cc
@@ -123,9 +123,7 @@ class RasterTaskImpl : public RasterTask {
         devtools_instrumentation::kRasterTask, layer_id_);
 
     skia::RefPtr<SkCanvas> canvas = raster_buffer_->AcquireSkCanvas();
-    if (!canvas)
-      return;
-    canvas->save();
+    DCHECK(canvas);
 
     skia::RefPtr<SkDrawFilter> draw_filter;
     switch (raster_mode_) {
@@ -164,7 +162,6 @@ class RasterTaskImpl : public RasterTask {
           100);
     }
 
-    canvas->restore();
     raster_buffer_->ReleaseSkCanvas(canvas);
   }