[remoting host] Fix div-by-zero in LeakyBucket

When receiving a bandwidth estimate of 0 from WebRTC, the host should
pause the video stream without crashing. This CL is a simple fix which
sets the bucket-emptying time to 1 minute if the rate is set to 0. This
is similar to what we already do in
FrameProcessingTimeEstimator::EstimatedTransitTime().

Bug: 806918
Change-Id: I43bf24cf54250cc3856adeb4c49809733320d46b
Reviewed-on: https://chromium-review.googlesource.com/891671Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532567}

remoting/base/leaky_bucket.cc

@@ -27,9 +27,17 @@ bool LeakyBucket::RefillOrSpill(int drops, base::TimeTicks now) {
 }
 
 base::TimeTicks LeakyBucket::GetEmptyTime() {
-  return level_updated_time_ +
-         base::TimeDelta::FromMicroseconds(
-             base::TimeTicks::kMicrosecondsPerSecond * current_level_ / rate_);
+  // To avoid unnecessary complexity in WebrtcFrameSchedulerSimple, we return
+  // a fairly large value (1 minute) here if the b/w estimate is 0 (which means
+  // that the video stream should be paused). This means that
+  // WebrtcFrameSchedulerSimple does not need to handle any overflow isssues
+  // caused by returning TimeDelta::Max().
+  base::TimeDelta time_to_empty =
+      (rate_ != 0) ? base::TimeDelta::FromMicroseconds(
+                         base::TimeTicks::kMicrosecondsPerSecond *
+                         current_level_ / rate_)
+                   : base::TimeDelta::FromMinutes(1);
+  return level_updated_time_ + time_to_empty;
 }
 
 void LeakyBucket::UpdateRate(int new_rate, base::TimeTicks now) {