Add RawPtr constructor taking a reference argument.

This fixed build failures with ENABLE_OILPAN after [1].

[1] http://src.chromium.org/viewvc/blink?view=revision&revision=168422

BUG=
TBR=haraken@chromium.org, ch.dumez@samsung.com

Review URL: https://codereview.chromium.org/187283006

git-svn-id: svn://svn.chromium.org/blink/trunk@168433 bbb929c8-8fbe-4397-9dbb-9b2b20218538