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A variable is initialized inside function MediaSessionPositionChanged
to temporally store duration/length information or remain 0 if duration
is not updated. This blocking the media position from updating as well.

Fixing the issue by add a zero check for the duration before compare to
the position.

BUG=b:147365793

Change-Id: I51d853bf8d618e28ccf75b68cea3c1d02f03b031
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095413
Commit-Queue: Jenny Zhang <jennyz@chromium.org>
Auto-Submit: Michael Sun <michaelfsun@google.com>
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748772}