cc: Use compositor context for signal sync token callback.

cc/raster/gpu_raster_buffer_provider.cc

Index: cc/raster/gpu_raster_buffer_provider.cc
diff --git a/cc/raster/gpu_raster_buffer_provider.cc b/cc/raster/gpu_raster_buffer_provider.cc
index c2b8c386914d3f2ca034a88c69cfbfbead7f8bed..408c72a58bee1d7d6977a484f29b9c3be598bce5 100644
--- a/cc/raster/gpu_raster_buffer_provider.cc
+++ b/cc/raster/gpu_raster_buffer_provider.cc
@@ -217,12 +217,13 @@ uint64_t GpuRasterBufferProvider::SetReadyToDrawCallback(
   DCHECK_NE(callback_id, 0u);
 
   // If the callback is different from the one the caller is already waiting on,
-  // pass the callback through to SignalSinkToken. Otherwise the request is
+  // pass the callback through to SignalSyncToken. Otherwise the request is
   // redundant.
   if (callback_id != pending_callback_id) {
-    // SignalSyncToken is threadsafe, no need for worker context lock.
-    worker_context_provider_->ContextSupport()->SignalSyncToken(sync_token,
-                                                                callback);
+    // Use the compositor context because we want this callback on the impl
+    // thread.
+    compositor_context_provider_->ContextSupport()->SignalSyncToken(sync_token,
+                                                                    callback);
   }
 
   return callback_id;