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- RTP timestamps in the maps of EncodingEventSubscribers (map keys, rtp timestamp fields in protos) are now relative to the first RTP timestamp seen since last Reset().
- The two GetXXXEventsAndReset() functions have been combined into one. In addition, it will now return the first RTP timestamp seen since the last Reset(). It will also reset the first RTP timestamp seen.
- There are two motivations for this change:
-- 1. RTP timestamps wrap around easily - around every 12 hours or so. If we do not use relative RTP timestamps, the ordering in the map, as well as general order of the events by frame, will be messed up.
-- 2. Using relative values will be more compact in variable-length encoding of the RTP timestamp fields.

Review URL: https://codereview.chromium.org/168933002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251973 0039d316-1c4b-4281-b951-d872f2087c98