Add ability to pass through IDWriteFactory so sandbox can control how it's constructed

Skia supports receiving a factory after Skia r14000. Plumb
a pointer through here so that the sandbox warmup code, see
https://codereview.chromium.org/217813008 can control how
it's constructed.

R=eae@chromium.org
BUG=333029

Review URL: https://codereview.chromium.org/213223003

git-svn-id: svn://svn.chromium.org/blink/trunk@170669 bbb929c8-8fbe-4397-9dbb-9b2b20218538