Fix crash when the OfflineAudioContext constructor is given 0 as numberOfFrames

Fix crash when the OfflineAudioContext constructor is given 0 as numberOfFrames.
The constructor now throws an exception in this case since we cannot construct
an AudioBuffer with zero frames.

R=kbr@chromium.org
BUG=306536
TEST=webaudio/offlineaudiocontext-constructor.html

Review URL: https://codereview.chromium.org/27039002

git-svn-id: svn://svn.chromium.org/blink/trunk@159499 bbb929c8-8fbe-4397-9dbb-9b2b20218538