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}
2 files changed