Remove check from SharedStyleFinder::canShareStyleWithElement

Since we know both elements have the same tag name at this
point, they also should return the same for isFormControlElement.
So weaken the check to an ASSERT.

Review URL: https://codereview.chromium.org/418573003

git-svn-id: svn://svn.chromium.org/blink/trunk@178834 bbb929c8-8fbe-4397-9dbb-9b2b20218538