Use monotonicallyIncreasingTime for Timeline timestamps

We used to use pseudo-wall time for Timeline timestamps and had
a hard time converting back-n-forth between monotonicallyIncreasingTime
used by other subsystems and Timeline. This switches Timeline to
monotonicallyIncreasingTime.

BUG=

Review URL: https://codereview.chromium.org/212893002

git-svn-id: svn://svn.chromium.org/blink/trunk@170444 bbb929c8-8fbe-4397-9dbb-9b2b20218538