Remove AudioBuffer::set_duration(), instead base on frames.

Remove AudioBuffer::set_duration(), instead base on frames.

wolenetz@ asked on https://codereview.chromium.org/251893002/ why
we allow set_duration() to be called.  It used to be required since
AudioBuffers had no concept of sample rate.  However, we now attach
a sample rate to every AudioBuffer which allows us to enforce a
valid duration for every buffer.

This CL also aligns MakeAudioBuffer() to do the same thing with
planar and interleaved data for the sake of easier testing.

BUG=none
TEST=media_unittests
NOTRY=true

Review URL: https://codereview.chromium.org/261533002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267779 0039d316-1c4b-4281-b951-d872f2087c98