Merge 21937 - Nice and easy fix for the media clock interpolating past the du...

Merge 21937 - Nice and easy fix for the media clock interpolating past the duration.

I considered adding duration to Clock itself, but that felt like it would pollute Clock's simple purposes of keeping track of time.  Furthermore, by keeping duration checking in PipelineImpl we can handle future scenarios such as streaming media where clock really should run forever.

BUG=16508
TEST=currentTime should never exceed duration

Review URL: http://codereview.chromium.org/159573

TBR=scherkus@chromium.org


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=22807