Change eventTime field to standard chrome.* API format.

Other timestamps in chrome.* use milliseconds since JS epoch. There
isn't any good reason to deviate from that. So we don't.

https://codereview.chromium.org/11299115/ brought up this issue, and
unfortunately I let it languish long enough that that CL isn't
easily mergeable anymore. Starting over. (thx dharcourt for getting
the ball rolling.)

BUG=161038


Review URL: https://chromiumcodereview.appspot.com/12335024

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@184137 0039d316-1c4b-4281-b951-d872f2087c98
5 files changed