Make getProperty more strict when handling the priority parameter

Make getProperty more strict when handling the priority parameter. Before this patch
only a substring search on "important" was done. However CSSOM is quite clear to be more strict :

http://dev.w3.org/csswg/cssom/#dom-cssstyledeclaration-setproperty
5. If priority is not the empty string and is not an ASCII case-insensitive match for the string "important", terminate this algorithm.

This patch implements that more strict behavior and adds a test.

Review URL: https://codereview.chromium.org/180353002

git-svn-id: svn://svn.chromium.org/blink/trunk@167883 bbb929c8-8fbe-4397-9dbb-9b2b20218538