Drop frames until specified bitrate is achieved.

This CL fixes a regression introduced with the new quality scaler
where the video would no longer start in a scaled mode. This CL adds
code that compares incoming captured frames to the target bitrate,
and if they are found to be too large, they are dropped and sinkWants
set to a lower resolution. The number of dropped frames should be low
(0-4 in most cases) and should not introduce a noticeable delay, or
at least should be preferrable to having the first 2-4 seconds of video
have very low quality.

BUG=webrtc:6953

Review-Url: https://codereview.webrtc.org/2630333002
Cr-Commit-Position: refs/heads/master@{#16391}

webrtc/modules/video_coding/utility/default_video_bitrate_allocator.cc

@@ -35,7 +35,6 @@ BitrateAllocation DefaultVideoBitrateAllocator::GetAllocation(
   } else {
     allocation.SetBitrate(0, 0, total_bitrate_bps);
   }
-
   return allocation;
 }