Enable style sharing for <option> and <optgroup>

This was attempted long ago but then reverted because it broke some tests
related to text transforms and disabled states [1]. Those tests don't seem
to fail anymore though and I can't think of any reason why sharing style
should change the rendering of RenderMenuList.

Enabling this makes http://jsfiddle.net/p4saV/1/ 56% faster where you have
5000 <option>'s and all of bootstrap's CSS.

[1] https://bugs.webkit.org/show_bug.cgi?id=88405

Review URL: https://codereview.chromium.org/215723002

git-svn-id: svn://svn.chromium.org/blink/trunk@170249 bbb929c8-8fbe-4397-9dbb-9b2b20218538