net-internals:  Use the time of the first event as the base time.

This makes time numbers more easily human-parseable.

BUG=327784
R=eroman@chromium.org

Review URL: https://codereview.chromium.org/98613004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242221 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242221