Speculative fix for bad SerialConnection behavior

If a SerialConnection consumer calls StartPolling() and then eventually
SetPaused(true), they should reasonably be able to assume that the
callback given to StartPolling() will no longer fire.

This changes SerialConnection::SetPaused() to ensure that any pending
timeout task is cancelled when pausing, satisfying the above assumption
where it was not previously satisfied.

This should fix a memory bug in ViscaWebcam which has been causing
crashes on Chrome OS.

Fixed: 1028819
Change-Id: I54c233fbc90fc2996d624548326ccda624fc9383
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974732Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#726271}

extensions/browser/api/serial/serial_connection.cc

@@ -203,7 +203,10 @@ void SerialConnection::SetPaused(bool paused) {
     return;
 
   paused_ = paused;
-  if (!paused_) {
+  if (paused_) {
+    // Make sure no pending timeout task fires.
+    receive_timeout_task_.Cancel();
+  } else {
     // If |receive_pipe_| is closed and there is no pending ReceiveError event,
     // try to reconnect the data pipe.
     if (!receive_pipe_ && !read_error_) {