Improve 'MutationObserver' exception error messages.

This patch drops a static 'validateOptions' method from 'MutationObserver',
as it was only ever used in one place, and it would have been more work to
pass an error message around than to just inline the 4 checks into
'MutationObserver::observe'.

BUG=270033
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/26801002

git-svn-id: svn://svn.chromium.org/blink/trunk@159333 bbb929c8-8fbe-4397-9dbb-9b2b20218538