aw: Allow blocking on viz process for webview

This CL adds exception for VizCompositorThreadRunnerWebView to use base
sync primitives to allow blocking UI/Android Renderer thread on viz
thread for viz for webview implementation.

Bug: 805739
Change-Id: I25d6b7494d6b722f1f1d1c6faaff84b552279d06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1911386Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714679}

android_webview/browser/gfx/viz_compositor_thread_runner_webview.cc

@@ -80,6 +80,7 @@ void VizCompositorThreadRunnerWebView::ScheduleOnVizAndBlock(
 void VizCompositorThreadRunnerWebView::PostTaskAndBlock(
     const base::Location& from_here,
     base::OnceClosure task) {
+  base::ScopedAllowBaseSyncPrimitivesOutsideBlockingScope allow_wait;
   base::WaitableEvent e;
   task_runner()->PostTask(from_here,
                           base::BindOnce(&RunAndSignal, std::move(task), &e));

base/threading/thread_restrictions.h

@@ -105,6 +105,7 @@ namespace android_webview {
 class AwFormDatabaseService;
 class CookieManager;
 class ScopedAllowInitGLBindings;
+class VizCompositorThreadRunnerWebView;
 }
 namespace audio {
 class OutputDevice;
@@ -461,6 +462,7 @@ class BASE_EXPORT ScopedAllowBaseSyncPrimitivesOutsideBlockingScope {
   friend class android_webview::
       AwFormDatabaseService;  // http://crbug.com/904431
   friend class android_webview::CookieManager;
+  friend class android_webview::VizCompositorThreadRunnerWebView;
   friend class audio::OutputDevice;
   friend class base::sequence_manager::internal::TaskQueueImpl;
   friend class base::FileDescriptorWatcher;