Remove ParseSVGAsHTML and tighten Document* types

This change removes the ParseSVGAsHTML feature as I am not currently
working on it. This is effectively a revert of
https://src.chromium.org/viewvc/blink?view=rev&revision=151572 with
a few additional includes that were removed in the interim.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23503026

git-svn-id: svn://svn.chromium.org/blink/trunk@157150 bbb929c8-8fbe-4397-9dbb-9b2b20218538