Initialize min/max record time to 0 instead of -1 or null

This simplifies many boundary time computations all around the code.

BUG=361045

Review URL: https://codereview.chromium.org/322783002

git-svn-id: svn://svn.chromium.org/blink/trunk@175801 bbb929c8-8fbe-4397-9dbb-9b2b20218538