Backport some tests in /content/public/test to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/content/public/test.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

Edit: Revert changes to RenderViewTest and clarify comments in
      RenderViewHostTestEnabler in terms of "TaskEnvironment", not
      "MessageLoop".

R=tommi@chromium.org

Bug: 891670
Change-Id: I019f7a9f8c7b07e922feffef1e271a8d2e35e291
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787066
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Tommi <tommi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694585}

content/public/test/test_renderer_host.cc

@@ -111,12 +111,14 @@ RenderViewHostTestEnabler::RenderViewHostTestEnabler()
       rvh_factory_(new TestRenderViewHostFactory(rph_factory_.get())),
       rfh_factory_(new TestRenderFrameHostFactory()),
       rwhi_factory_(new TestRenderWidgetHostFactory()) {
-  // A MessageLoop is needed for Mojo bindings to graphics services. Some
-  // tests have their own, so this only creates one when none exists. This
-  // means tests must ensure any MessageLoop they make is created before
-  // the RenderViewHostTestEnabler.
-  if (!base::MessageLoopCurrent::Get())
-    task_environment_ = std::make_unique<base::test::TaskEnvironment>();
+  // A TaskEnvironment is needed on the main thread for Mojo bindings to
+  // graphics services. Some tests have their own, so this only creates one
+  // (single-threaded) when none exists. This means tests must ensure any
+  // TaskEnvironment they make is created before the RenderViewHostTestEnabler.
+  if (!base::MessageLoopCurrent::Get()) {
+    task_environment_ =
+        std::make_unique<base::test::SingleThreadTaskEnvironment>();
+  }
 #if !defined(OS_ANDROID)
   ImageTransportFactory::SetFactory(
       std::make_unique<TestImageTransportFactory>());

content/public/test/test_renderer_host.h

@@ -154,7 +154,7 @@ class RenderViewHostTestEnabler {
 #if defined(OS_ANDROID)
   std::unique_ptr<display::Screen> screen_;
 #endif
-  std::unique_ptr<base::test::TaskEnvironment> task_environment_;
+  std::unique_ptr<base::test::SingleThreadTaskEnvironment> task_environment_;
   std::unique_ptr<MockRenderProcessHostFactory> rph_factory_;
   std::unique_ptr<TestRenderViewHostFactory> rvh_factory_;
   std::unique_ptr<TestRenderFrameHostFactory> rfh_factory_;