Cast: Use difference encoding on RTP timestamps for serializing events

Cast: Use difference encoding on RTP timestamps for serializing events

Previously during serialization of raw event logs we write out RTP
timestamp of each frame as a relative value to the first RTP timestamp
seen. This number can get large for later frames. This patch changes
the serialization logic so it writes out the RTP timestamp of each
frame as a relative value to the previous frame. This results in smaller
values being written out for each frame, which results in a smaller
serialization length due to variable length encoding.

The result of this change is that serialized length with compression is
now 19% shorter. (86.5kb vs 70kb for 30 seconds of data)

Test: SerializeDeserializeTest still passes

BUG=338574,301920

Review URL: https://codereview.chromium.org/195943011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257035 0039d316-1c4b-4281-b951-d872f2087c98