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The previous fix for positive start times http://crrev.com/93c37f1b is
incomplete. It fixed seeking, but did not fix rendering. As such, the
media timeline will start playback from 0 and advance in real time to
any positive start time... even if the actual start is hours away.

When testing I used a test clip with a short positive start time so
did not notice the issue.  In attempting to write a layout test, I
retested with a larger start time which revealed the deficiency in my
first solution.

This change reverts the removal of Demuxer::StartTime() with the added
restriction that it must always be positive.  Pipeline uses this time
like it did before, during initialization and as a minimum bound for
seeking.

BUG=413292
TEST=four new tests! layout test forthcoming.

Review URL: https://codereview.chromium.org/575643002

Cr-Commit-Position: refs/heads/master@{#295159}