Prefer ScopedNestableTaskAllower over manual save/restore
Cleanup. Changes various calls sites to use the
ScopedNestableTaskAllower class to save/restore nestable task
state.
BUG=None
TEST=Existing unit tests
R=jar@chromium.org, scottbyer@chromium.org, sky@chromium.org, akalin@chromium.org, rsleevi@chromium.org, brettw@chromium.org, tony@chromium.org
Review URL: https://chromiumcodereview.appspot.com/9384024
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@121914 0039d316-1c4b-4281-b951-d872f2087c98