[XHR] remove now-unnecessary RaisesException attributes.

A handful of getters and methods now follow the spec and no longer
throw: synchronize their signatures and underlying IDL to reflect that
by removing [RaisesException] use.

R=tyoshino@chromium.org
BUG=

Review URL: https://codereview.chromium.org/108573002

git-svn-id: svn://svn.chromium.org/blink/trunk@163491 bbb929c8-8fbe-4397-9dbb-9b2b20218538