Makes BitmapUploader handle null GLES2Context

components/bitmap_uploader/bitmap_uploader.h

Index: components/bitmap_uploader/bitmap_uploader.h
diff --git a/components/bitmap_uploader/bitmap_uploader.h b/components/bitmap_uploader/bitmap_uploader.h
index bd1b70d4f5bb08d261a79152564de2674ed8087a..d33bc834045aa0e2abc3f0199f83f111847f9777 100644
--- a/components/bitmap_uploader/bitmap_uploader.h
+++ b/components/bitmap_uploader/bitmap_uploader.h
@@ -73,6 +73,8 @@ class BITMAP_UPLOADER_EXPORT BitmapUploader
 
   mus::Window* window_;
   std::unique_ptr<mus::WindowSurface> surface_;
+  // This may be null if there is an error contacting mus/initializing. We
+  // assume we'll be shutting down soon and do nothing in this case.
   std::unique_ptr<mus::GLES2Context> gles2_context_;
 
   uint32_t color_;