Have MixedContentChecker and ProgressTracker keep a LocalFrame Member.

Tidy up the Oilpan handling of these two FrameLoader-owned objects,
and arrange it so that their LocalFrame back references are correctly
traced.

R=haraken,mkwst
BUG=340522

Review URL: https://codereview.chromium.org/596653006

git-svn-id: svn://svn.chromium.org/blink/trunk@182652 bbb929c8-8fbe-4397-9dbb-9b2b20218538