[remoting host] Rate-limit capturer to 30FPS

For WebRTC-enabled connections, ensure capturer is not scheduled more
often than 30FPS, to limit CPU usage.

Bug: 816727
Change-Id: If2efd9de0749848f209fad7ed1926be108f402c3
Reviewed-on: https://chromium-review.googlesource.com/940449Reviewed-by: Zijie He <zijiehe@chromium.org>
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539957}

remoting/protocol/webrtc_frame_scheduler_simple.cc

@@ -272,6 +272,11 @@ void WebrtcFrameSchedulerSimple::ScheduleNextFrame() {
         kTargetFrameInterval -
         estimated_next_frame_cost;
     target_capture_time = std::max(target_capture_time, ideal_capture_time);
+
+    // Ensure that the capture rate is capped by kTargetFrameInterval, to avoid
+    // excessive CPU usage by the capturer.
+    target_capture_time = std::max(
+        target_capture_time, last_capture_started_time_ + kTargetFrameInterval);
   }
 
   target_capture_time = std::max(target_capture_time, now);