Move modules-dependent eventtarget code out of core.
This is a follow-up CL on https://codereview.chromium.org/275283002/

BUG=371581

Review URL: https://codereview.chromium.org/297493004

git-svn-id: svn://svn.chromium.org/blink/trunk@174387 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Use tighter typing for RenderTextFragment::firstLetter()

Use tighter typing for RenderTextFragment::firstLetter(). We can use a
RenderBoxModelObject* instead of a simpler RenderObject*. There is no
performance impact at this point as RenderBoxModelObject does not
have firstChild() / lastChild() and RenderObject's slowFirstChild() /
slowLastChild() are still being called.

R=esprehn@chromium.org, pdr@chromium.org

Review URL: https://codereview.chromium.org/295923002

git-svn-id: svn://svn.chromium.org/blink/trunk@174388 bbb929c8-8fbe-4397-9dbb-9b2b20218538