Signal exception when assigning invalid value to WaveShaper oversample.

BUG=299199

Remove the OverSampleType from the idl.

Add test to verify exception is thrown.

Review URL: https://codereview.chromium.org/24890002

git-svn-id: svn://svn.chromium.org/blink/trunk@158760 bbb929c8-8fbe-4397-9dbb-9b2b20218538