Record Uptime in UMA Logs

Adds a new field in system_profile.proto for total uptime since startup;
renames existing field to reflect its actual status as incremental uptime
since last log was written.

The internal proto changes for this have already landed.  The changes
are copied wholesale into this patch with the exception of adding a statement
about when chromium started logging uptime.  (I will upstream that comment
sometime after this change is submitted.)

Tested on Linux with a LOG(INFO) line in metrics_log.cc to display the
uptimes it writes to the logs

BUG=349568

Review URL: https://codereview.chromium.org/195563004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256672 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256672