[webcodecs] Fix crash in VideoFrame.createImageBitmap() in workers

Without access to the shared main thread context,
VideoFrame.createImageBitmap() is unable to handle texture-backed
frames.

This CL does not fix the issue, but it does change it from a check
failure to a DOMException.

Bug: 1148849
Change-Id: I3f8c292022dc3cb8fd6625591d2384511e253ac2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2552901
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Auto-Submit: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829865}

third_party/blink/renderer/modules/webcodecs/video_frame.cc

@@ -438,6 +438,16 @@ ScriptPromise VideoFrame::CreateImageBitmap(ScriptState* script_state,
           SkImage::MakeRasterData(info, image_pixels, bytes_per_row);
       image = UnacceleratedStaticBitmapImage::Create(std::move(skImage));
     } else {
+      if (!IsMainThread()) {
+        // TODO(crbug.com/1148849): We should either hop to the media thread and
+        // use the media context, or use the SharedGpuContext. Currently
+        // obtaining the media context requires synchronizing with the main
+        // thread, and PaintCanvasVideoRenderer is not compatible with
+        // SharedGpuContext.
+        exception_state.ThrowDOMException(DOMExceptionCode::kOperationError,
+                                          "Graphics context unavailable.");
+        return ScriptPromise();
+      }
       scoped_refptr<viz::RasterContextProvider> raster_context_provider =
           Platform::Current()->SharedMainThreadContextProvider();
       auto* ri = raster_context_provider->RasterInterface();