scheduler: Avoid using default task runner at MockFetchContext

This is a part of efforts to forbid V8 execution on the default task
runner of the main thread.

Bug: 870606

Change-Id: I3aa11f7cb4f8d6da6b0015d8b5516b1b2ba53bbf
Reviewed-on: https://chromium-review.googlesource.com/1169037Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582120}

third_party/blink/renderer/platform/loader/fetch/resource_fetcher_test.cc

@@ -95,6 +95,12 @@ class ResourceFetcherTest : public testing::Test {
   ResourceFetcherTest() = default;
   ~ResourceFetcherTest() override { GetMemoryCache()->EvictResources(); }
 
+  void RunUntilIdle() {
+    base::SingleThreadTaskRunner* runner =
+        Context()->GetLoadingTaskRunner().get();
+    static_cast<scheduler::FakeTaskRunner*>(runner)->RunUntilIdle();
+  }
+
  protected:
   MockFetchContext* Context() { return platform_->Context(); }
   void AddResourceToMemoryCache(
@@ -905,7 +911,7 @@ TEST_F(ResourceFetcherTest, StaleWhileRevalidate) {
   new_resource = MockResource::Fetch(fetch_params, fetcher, nullptr);
   EXPECT_EQ(resource, new_resource);
   EXPECT_TRUE(GetMemoryCache()->Contains(resource));
-  platform_->RunUntilIdle();
+  RunUntilIdle();
   platform_->GetURLLoaderMockFactory()->ServeAsynchronousRequests();
   EXPECT_FALSE(GetMemoryCache()->Contains(resource));
 }

third_party/blink/renderer/platform/loader/testing/fetch_testing_platform_support.cc

@@ -30,8 +30,7 @@ FetchTestingPlatformSupport::~FetchTestingPlatformSupport() {
 MockFetchContext* FetchTestingPlatformSupport::Context() {
   if (!context_) {
     context_ =
-        MockFetchContext::Create(MockFetchContext::kShouldLoadNewResource,
-                                 scheduler_->DefaultTaskRunner());
+        MockFetchContext::Create(MockFetchContext::kShouldLoadNewResource);
   }
   return context_;
 }