Add appropriate enum types for string attributes.

Also, setting invalid values for channelCountMode, channelInterpretation, and oversample should not signal errors.  
Using these enum types seems to prevent reaching the code that signals the exceptions so remove that code too.

Updated dom-exceptions test to verify that exceptions are not signaled and that the values are not changed.

BUG=371412

Review URL: https://codereview.chromium.org/279473004

git-svn-id: svn://svn.chromium.org/blink/trunk@174031 bbb929c8-8fbe-4397-9dbb-9b2b20218538