[media] Fix video frame copy when context is lost.

If a GPU channel cannot be established, a WebMediaPlayerImpl can be
constructed without a |context_provider_|. Handle that case correctly
in CopyVideoTextureToPlatformTexture().

Bug: 819914
Change-Id: I620b602ee0c092cc31273b4f513133bf8af7e1d0
Reviewed-on: https://chromium-review.googlesource.com/1250137
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Fredrik Hubinette <hubbe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594939}

media/blink/webmediaplayer_impl.cc

@@ -1239,14 +1239,15 @@ bool WebMediaPlayerImpl::CopyVideoTextureToPlatformTexture(
   }
 
   Context3D context_3d;
+  gpu::ContextSupport* context_support = nullptr;
   if (context_provider_) {
     context_3d = Context3D(context_provider_->ContextGL(),
                            context_provider_->GrContext());
+    context_support = context_provider_->ContextSupport();
   }
   return video_renderer_.CopyVideoFrameTexturesToGLTexture(
-      context_3d, context_provider_->ContextSupport(), gl, video_frame.get(),
-      target, texture, internal_format, format, type, level, premultiply_alpha,
-      flip_y);
+      context_3d, context_support, gl, video_frame.get(), target, texture,
+      internal_format, format, type, level, premultiply_alpha, flip_y);
 }
 
 // static