Fix dragging text inside a input element nested in a draggable element.

I'm not entirely sure why this used to work and was broken by my
DragController refactoring. Manual inspection seemed to indicate that
it shouldn't have worked before the refactoring either, but it was
probably a combination of several different interactions that allowed
it to work. With this change, we (mostly) match what the spec says we
should do, and more importantly, we match Firefox's behavior. As a
bonus, the code is somewhat simpler as well.

BUG=298659

Review URL: https://codereview.chromium.org/50433002

git-svn-id: svn://svn.chromium.org/blink/trunk@161750 bbb929c8-8fbe-4397-9dbb-9b2b20218538