Added WebRTC.Video.H264Encoder/DecoderImpl.Event histograms.

These histograms will be used to track how many times
this encoder/decoder is initialized/an error occurs.

WebRTC sibling CL:
https://codereview.webrtc.org/1716173002/

BUG=chromium:500605, chromium:468365

Review URL: https://codereview.chromium.org/1719273002

Cr-Commit-Position: refs/heads/master@{#377541}
1 file changed