Flush TaskScheduler in ~ScopedTaskEnvironment and ~ScopedAsyncTaskScheduler.

Skipping tasks when destroying ScopedTaskEnvironment or
ScopedAsyncTaskScheduler means that DeleteSoon() tasks
can be skipped, resulting in memory leaks.

BUG=708584, 709095

Review-Url: https://codereview.chromium.org/2812293002
Cr-Commit-Position: refs/heads/master@{#464380}

base/test/scoped_async_task_scheduler.cc

@@ -29,6 +29,9 @@ ScopedAsyncTaskScheduler::ScopedAsyncTaskScheduler() {
 
 ScopedAsyncTaskScheduler::~ScopedAsyncTaskScheduler() {
   DCHECK_EQ(TaskScheduler::GetInstance(), task_scheduler_);
+  // Without FlushForTesting(), DeleteSoon() and ReleaseSoon() tasks could be
+  // skipped, resulting in memory leaks.
+  TaskScheduler::GetInstance()->FlushForTesting();
   TaskScheduler::GetInstance()->Shutdown();
   TaskScheduler::GetInstance()->JoinForTesting();
   TaskScheduler::SetInstance(nullptr);

base/test/scoped_task_environment.cc

@@ -32,6 +32,9 @@ ScopedTaskEnvironment::~ScopedTaskEnvironment() {
   RunLoop().RunUntilIdle();
 
   DCHECK_EQ(TaskScheduler::GetInstance(), task_scheduler_);
+  // Without FlushForTesting(), DeleteSoon() and ReleaseSoon() tasks could be
+  // skipped, resulting in memory leaks.
+  TaskScheduler::GetInstance()->FlushForTesting();
   TaskScheduler::GetInstance()->Shutdown();
   TaskScheduler::GetInstance()->JoinForTesting();
   TaskScheduler::SetInstance(nullptr);