ozone: scenic: Use IO message loop for GPU

This is necessary in order to be able to call FIDL interfaces from the
GPU thread.

Bug: 861853
Test: run_content_shell --ozone-platform=scenic --disable-gpu --enable-vulkan

Change-Id: I1f7b0f2957a810745a83ce1cf36327467e6ea448

Reviewed-on: https://chromium-review.googlesource.com/c/1330290
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607314}

ui/ozone/platform/scenic/ozone_platform_scenic.cc

@@ -127,6 +127,11 @@ class OzonePlatformScenic
 
   void InitializeGPU(const InitParams& params) override {}
 
+  base::MessageLoop::Type GetMessageLoopTypeForGpu() override {
+    // Scenic FIDL calls require async dispatcher.
+    return base::MessageLoop::TYPE_IO;
+  }
+
  private:
   // Performs graceful cleanup tasks on main message loop teardown.
   void Shutdown() { window_manager_.reset(); }