Cast: Added LogMetadata to raw_events.proto to hold metadata.

Metadata contains things we were writing to the file separately before:
- is_audio
- first_rtp_timestamp
- num_frame_events
- num_packet_events

By bundling these things into a metadata proto, the overall log file
format becomes more flexible to changes.

BUG=301920,338574

Review URL: https://codereview.chromium.org/189143003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256112 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256112