[MBI] Migrate WebViewPlugin to use per-ASG CompositorTaskRunner

Before this CL, WebViewPlugin uses per-thread CompositorTaskRunner.

After this CL, WebViewPlugin will use per-ASG CompositorTaskRunner.

Bug: 1105403
Change-Id: Ie5480f35818e3821799a20785f2bde5b4cb62d17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626679
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843924}

components/plugins/renderer/webview_plugin.cc

@@ -193,7 +193,8 @@ void WebViewPlugin::UpdateGeometry(const WebRect& window_rect,
 
   // Plugin updates are forbidden during Blink layout. Therefore,
   // UpdatePluginForNewGeometry must be posted to a task to run asynchronously.
-  blink::scheduler::WebThreadScheduler::MainThreadScheduler()
+  web_view_helper_.main_frame()
+      ->GetAgentGroupScheduler()
       ->CompositorTaskRunner()
       ->PostTask(FROM_HERE,
                  base::BindOnce(&WebViewPlugin::UpdatePluginForNewGeometry,