[Cast MRP] Stop mirroring when Chrome shuts down.

Bug: b/161367720
Change-Id: I619b88eec584b2124dcab6434e59f45cfe0e9c2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300661
Commit-Queue: John Williams <jrw@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793279}

chrome/browser/media/router/providers/cast/mirroring_activity.cc

@@ -133,6 +133,14 @@ MirroringActivity::MirroringActivity(
 }
 
 MirroringActivity::~MirroringActivity() {
+  // Usually mirroring will have already been stopped by this point, but when
+  // Chrome is shutting down, this is our first indication that mirroring needs
+  // to stop.  Without this call, if a user shuts down Chrome while mirroring,
+  // the receiver will continue to show the last mirrored frame for several
+  // seconds, which could be bad if the user was shutting down Chrome
+  // specifically to prevent someone from seeing what they were mirroring!
+  StopMirroring();
+
   if (did_start_mirroring_timestamp_) {
     base::UmaHistogramLongTimes(
         kHistogramSessionLength,