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As a new name for ScopedNestableTaskAllower. A migration will follow
to move callers either to the new name or to
RunLoop::Type::kNestableTasksAllowed.

R=fdoray@chromium.org

Bug: 781352
Change-Id: I5208655072728b4305e70815ccb933173ff0842e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225834
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775160}