Don't rebase timestamps for positive start times.
This is wrong according to the HTML Media specification:
http://dev.w3.org/html5/spec-preview/media-elements.html#offsets-into-the-media-resource
"If the media resource somehow specifies an explicit timeline whose
origin is not negative (i.e. gives each frame a specific time offset
and gives the first frame a zero or positive offset), then the media
timeline should be that timeline."
BUG=413292
TEST=updated unittest.
Review URL: https://codereview.chromium.org/
562323002
Cr-Commit-Position: refs/heads/master@{#294461}