Reset frame timestamp epoch for new capturers.

Incoming frames usually have an epoch of time since the capturer was
created or similar, not any fixed-time epoch. As such, setting a new
capturer resulted in delivering frames with older timestamps which
caused these frames to be dropped before encoding.

BUG=webrtc:4994
R=stefan@webrtc.org
TBR=pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1345473002

Cr-Commit-Position: refs/heads/master@{#9934}
(cherry picked from commit 1cb121dea478a4bb4f88e76cf92719e2853543cf)

BUG=chromium:532132
TBR=tnakamura@webrtc.org

Review URL: https://codereview.webrtc.org/1352433002 .

Cr-Commit-Position: refs/branch-heads/46@{#3}
Cr-Branched-From: a1f590f3b6d317047d28b6568d1229973bb93f29-refs/heads/master@{#9745}