Previously the frame scheduler was setting target bitrate whenever
bandwidth estimate changes. VP8 encoder doesn't handle this very well:
every time bitrate is update quality drops significanlty for the
following frames. It happens because every time biutrate is updated the
codec needs to be reconfigured, resulting in encoder state reset.

Fixed it by adding a filter that calculates average BW estimate over
last second and updates target bitrate only when the average BW
estimate differs from the current target bitrate by more than 33%.

Also fixed WebrtcConnectionToHost::Connect() to avoid crash when
running perftests.

Review-Url: https://codereview.chromium.org/2562893003
Cr-Commit-Position: refs/heads/master@{#437747}