Have AUAudioInputStream::Stop() also cancel any deferred start.

BUG=160920
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2860653003
Cr-Commit-Position: refs/heads/master@{#469296}

media/audio/mac/audio_low_latency_input_mac.cc

@@ -577,6 +577,7 @@ void AUAudioInputStream::Start(AudioInputCallback* callback) {
 
 void AUAudioInputStream::Stop() {
   DCHECK(thread_checker_.CalledOnValidThread());
+  deferred_start_cb_.Cancel();
   DVLOG(1) << "Stop";
   StopAgc();
   if (check_alive_timer_ != nullptr) {