Oilpan: have DocumentInit use transition types throughout.

For DocumentInit, the context document is no longer mediated
as a WeakPtr, but as a Member. It is sufficient that the
Document holds onto this reference weakly.

R=haraken@chromium.org, tkent@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/304323010

git-svn-id: svn://svn.chromium.org/blink/trunk@175260 bbb929c8-8fbe-4397-9dbb-9b2b20218538