Notify Javascript about the size of an AudioBuffer

BUG=282683, 249161

A patch from Mark Buer (see
https://code.google.com/p/chromium/issues/detail?id=249161).

Report the actual size of the AudioBuffer to the garbage collector so
that creating these large buffers will trigger garbage collection.

Review URL: https://chromiumcodereview.appspot.com/23619014

git-svn-id: svn://svn.chromium.org/blink/trunk@157857 bbb929c8-8fbe-4397-9dbb-9b2b20218538