Accessible scrollToMakeVisible should avoid scrolling if subfocus is visible.

The code already avoided scrolling if an object was already visible.
This extends that logic so that if the object is larger than the viewport,
it avoids scrolling if the subfocus is already visible.

BUG=374316, 424691

Review URL: https://codereview.chromium.org/657903004

git-svn-id: svn://svn.chromium.org/blink/trunk@183932 bbb929c8-8fbe-4397-9dbb-9b2b20218538