Oilpan: add WillBeGarbageCollected to HTMLDocument idl file.

R=haraken@chromium.org
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/294023011

git-svn-id: svn://svn.chromium.org/blink/trunk@174545 bbb929c8-8fbe-4397-9dbb-9b2b20218538