Tweak ScopedTaskEnvironmentTest comments as per code-review requests.

Bug: 789712
TBR: gab
Change-Id: I16e059c18420a56e875c5564015531177bc213d3
Reviewed-on: https://chromium-review.googlesource.com/806815Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521434}

base/test/scoped_task_environment_unittest.cc

@@ -136,6 +136,8 @@ TEST_P(ScopedTaskEnvironmentTest,
 }
 
 TEST_P(ScopedTaskEnvironmentTest, DelayedTasks) {
+  // Use a QUEUED execution-mode environment, so that no tasks are actually
+  // executed until RunUntilIdle()/FastForwardBy() are invoked.
   ScopedTaskEnvironment scoped_task_environment(
       GetParam(), ScopedTaskEnvironment::ExecutionMode::QUEUED);
 
@@ -191,11 +193,8 @@ TEST_P(ScopedTaskEnvironmentTest, DelayedTasks) {
                           },
                           Unretained(&counter)));
 
-  // Verify that tasks are not being processed asynchronously. Since scheduling
-  // is non-deterministic, we pause the main thread momentarily, to increase
-  // the likelihood of the preceding PostTask() being run.
-  base::PlatformThread::Sleep(base::TimeDelta::FromMilliseconds(10));
-
+  // This expectation will fail flakily if the preceding PostTask() is executed
+  // asynchronously, indicating a problem with the QUEUED execution mode.
   int expected_value = 0;
   EXPECT_EQ(expected_value, counter);