[MBI] Remove unused dispatcher_task_runner variable

dispatcher_task_runner is not used.

Bug: 1105403
Change-Id: Id77a0ae73594597df75031b61588081bdb55040c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2624270Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842798}

third_party/blink/renderer/platform/graphics/canvas_resource_dispatcher.cc

@@ -140,8 +140,6 @@ void CanvasResourceDispatcher::PostImageToPlaceholderIfNotBlocked(
 void CanvasResourceDispatcher::PostImageToPlaceholder(
     scoped_refptr<CanvasResource> canvas_resource,
     viz::ResourceId resource_id) {
-  scoped_refptr<base::SingleThreadTaskRunner> dispatcher_task_runner =
-      Thread::Current()->GetTaskRunner();
   // After this point, |canvas_resource| can only be used on the main thread,
   // until it is returned.
   canvas_resource->Transfer();