Clean up code duplication of isAtShadowBoundary()

Moved isAtShadowBoundary() to Element.h in order to remove duplication.

BUG=

Review URL: https://codereview.chromium.org/525393002

git-svn-id: svn://svn.chromium.org/blink/trunk@181201 bbb929c8-8fbe-4397-9dbb-9b2b20218538