Add ability to clear pending tasks of TestMockTimeTaskRunner without running the tasks.

BUG=None

Review URL: https://codereview.chromium.org/994903002

Cr-Commit-Position: refs/heads/master@{#319896}

base/test/test_mock_time_task_runner.cc

@@ -100,6 +100,13 @@ void TestMockTimeTaskRunner::FastForwardUntilNoTasksRemain() {
   ProcessAllTasksNoLaterThan(TimeDelta::Max());
 }
 
+void TestMockTimeTaskRunner::ClearPendingTasks() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  AutoLock scoped_lock(tasks_lock_);
+  while (!tasks_.empty())
+    tasks_.pop();
+}
+
 Time TestMockTimeTaskRunner::Now() const {
   DCHECK(thread_checker_.CalledOnValidThread());
   return now_;

base/test/test_mock_time_task_runner.h

@@ -61,6 +61,9 @@ class TestMockTimeTaskRunner : public SingleThreadTaskRunner {
   // elapse.
   void RunUntilIdle();
 
+  // Clears the queue of pending tasks without running them.
+  void ClearPendingTasks();
+
   // Returns the current virtual time (initially starting at the Unix epoch).
   Time Now() const;