Don't allow sharing between elements that have different :host styles

Elements can only share style if they either both don't have any
ShadowRoots or they both have ShadowRoots and all their ShadowRoots have
the same styles.

We were incorrectly allowing sharing between elements without shadow roots
and ones with shadow roots, and also allowing sharing between elements that
had styles in an older ShadowRoot but no <shadow> in the younger shadow
root. The second case was previously correct before r170091 when we made
styles apply even when <shadow> was not present.

Review URL: https://codereview.chromium.org/236193006

git-svn-id: svn://svn.chromium.org/blink/trunk@171533 bbb929c8-8fbe-4397-9dbb-9b2b20218538