Use toElement instead of static_cast<Element*> and static_cast<const Element*>

toElement is already used in many places, however, there are still exist
static_cast<Element*> and static_cast<const Element*>. This CL replaces
these static_cast with toElement.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/16879016

git-svn-id: svn://svn.chromium.org/blink/trunk@152922 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove WebKit/chromium/public fowarding headers

After this CL lands, no one should be referring to the old location anymore.
The new location is /public/web.

BUG=239545
TBR=jamesr

Review URL: https://chromiumcodereview.appspot.com/17486002

git-svn-id: svn://svn.chromium.org/blink/trunk@152923 bbb929c8-8fbe-4397-9dbb-9b2b20218538