Invert logic that prevented style sharing in findSharedStyle().

https://src.chromium.org/viewvc/blink?view=rev&revision=159972 contained
a bug that prevented style sharing. This patch inverts the logic
so that if the parent supports style sharing, we do not return 0 from
SharedStyleFinder::findSharedStyle().

BUG=309935

Review URL: https://codereview.chromium.org/33183010

git-svn-id: svn://svn.chromium.org/blink/trunk@160172 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move some checks from SharedStyleFinder::canShareStyleWithElement to Element::supportsStyleSharing

The checks for tagName and full screen can be decided before we even
put the element into the candidate list. This reduces the number of
checks when looking for a sharing candidate and also opens spaces in
the candidate list for elements which can actually share since putting
<iframe> or other elements in the list was just wasting space and
evicting elements that possibly could have shared.

This may improve sharing on pages with many <option> elements since
previously we would have filled the sharing list with them even though
none of them can actually share. Now we'll skip even putting them in
the list so after recalcing style on a <select> with many elements we'll
still have a style sharing list full of elements to share with the elements
that are after the <select>.

Review URL: https://codereview.chromium.org/34153004

git-svn-id: svn://svn.chromium.org/blink/trunk@160173 bbb929c8-8fbe-4397-9dbb-9b2b20218538