Get rid of custom bindings for AudioBufferSourceNode.buffer setter

Get rid of custom bindings for AudioBufferSourceNode.buffer setter by
leveraging [StrictTypeChecking] IDL extended attribute and updating
the implementation to throw if the input buffer is NULL or if the
number of channels is too big.

There is no web-exposed behavior change.

R=haraken, kbr
BUG=

Review URL: https://codereview.chromium.org/26883005

git-svn-id: svn://svn.chromium.org/blink/trunk@159490 bbb929c8-8fbe-4397-9dbb-9b2b20218538