[Chromoting] Change ScreenRefresh to use 'if' instead of 'DCHECK'.

remoting/host/video_frame_capturer_mac.mm

Index: remoting/host/video_frame_capturer_mac.mm
===================================================================
--- remoting/host/video_frame_capturer_mac.mm	(revision 163603)
+++ remoting/host/video_frame_capturer_mac.mm	(working copy)
@@ -747,7 +747,9 @@
 
 void VideoFrameCapturerMac::ScreenRefresh(CGRectCount count,
                                           const CGRect* rect_array) {
-  DCHECK(!desktop_bounds_.isEmpty());
+  if (desktop_bounds_.isEmpty()) {
+    return;
+  }
   SkIRect skirect_array[count];
   for (CGRectCount i = 0; i < count; ++i) {
     skirect_array[i] = CGRectToSkIRect(rect_array[i]);