Have SubframeLoadingDisabler singleton live off-heap.

Primarily to diagnose potential heap corruption, keep
track of the disabled set off-heap. The untraced references
added aren't unsafe, as they are all stack-reachable by
construction.

R=dcheng,haraken
BUG=684551

Review-Url: https://codereview.chromium.org/2667853006
Cr-Commit-Position: refs/heads/master@{#447682}
2 files changed