Backport some tests in /content/renderer/pepper 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/renderer/pepper.

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

This CL was uploaded by git cl split.

R=bbudge@chromium.org

Bug: 891670
Change-Id: Idca36fd4180c5e8047274ffb141a074562413e71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786807
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693924}

content/renderer/pepper/pepper_device_enumeration_host_helper_unittest.cc

@@ -146,7 +146,7 @@ class PepperDeviceEnumerationHostHelperTest : public testing::Test {
   ppapi::host::PpapiHost ppapi_host_;
   ppapi::host::ResourceHost resource_host_;
   PepperDeviceEnumerationHostHelper device_enumeration_;
-  base::test::TaskEnvironment
+  base::test::SingleThreadTaskEnvironment
       task_environment_;  // required for async calls to work.
 
  private:

content/renderer/pepper/pepper_graphics_2d_host_unittest.cc

@@ -82,7 +82,7 @@ class PepperGraphics2DHostTest : public testing::Test {
  private:
   ppapi::ViewData renderer_view_data_;
   std::unique_ptr<PepperGraphics2DHost> host_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   MockRendererPpapiHost renderer_ppapi_host_;
   ppapi::TestGlobals test_globals_;
 };

content/renderer/pepper/plugin_instance_throttler_impl_unittest.cc

@@ -83,7 +83,7 @@ class PluginInstanceThrottlerImplTest
 
   int change_callback_calls_;
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 };
 
 TEST_F(PluginInstanceThrottlerImplTest, ThrottleAndUnthrottleByClick) {

content/renderer/pepper/v8_var_converter_unittest.cc

@@ -239,7 +239,7 @@ class V8VarConverterTest : public testing::Test {
   std::unique_ptr<V8VarConverter> converter_;
 
  private:
-  base::test::TaskEnvironment
+  base::test::SingleThreadTaskEnvironment
       task_environment_;  // Required to receive callbacks.
 
   TestGlobals globals_;