XMLHttpRequest::overrideMimeType should throw InvalidStateError

Before this CL, XMLHttpRequest accepted |overrideMimeType| regardless
of its state. This is not conformant to the current spec:
http://www.w3.org/TR/XMLHttpRequest2/#the-overridemimetype-method

This CL changes it to throw InvalidStateError when the state
is LOADING or DONE, which aligns to the spec.

BUG=402375

Review URL: https://codereview.chromium.org/478223002

git-svn-id: svn://svn.chromium.org/blink/trunk@180440 bbb929c8-8fbe-4397-9dbb-9b2b20218538