content: Use background priority for raster thread in browser compositor on Android.

BUG=470091

Review URL: https://codereview.chromium.org/1031163002

Cr-Commit-Position: refs/heads/master@{#322199}

content/browser/renderer_host/compositor_impl_android.cc

@@ -145,6 +145,7 @@ class SingleThreadTaskGraphRunner
  public:
   SingleThreadTaskGraphRunner() : worker_thread_(this, "CompositorWorker") {
     worker_thread_.Start();
+    worker_thread_.SetThreadPriority(base::kThreadPriority_Background);
   }
 
   ~SingleThreadTaskGraphRunner() override {