Stop throwing DOM exceptions in internal 'XMLHttpRequest' response getters.

Unlike 'responseText' and 'responseXML', 'responseBlob' and
'responseArrayBuffer' are not exposed to JavaScript (they don't
appear in the IDL or in the specification). As they are only called from
custom bindings in response to a JavaScript call to the 'response' getter,
we can safely replace the exception-throwing code in the implementation
with an ASSERT that the request type is correct.

Review URL: https://chromiumcodereview.appspot.com/22842002

git-svn-id: svn://svn.chromium.org/blink/trunk@155954 bbb929c8-8fbe-4397-9dbb-9b2b20218538