Allow timer heap updates on firing shared timer.
Allow timer heap updates on firing shared timer.

Follow up the ASan exemptions from r195613 and allow timer heap updates
while servicing the shared timer callback. The Timer implementation of
fired() already checks that no access of near-dead objects is done.

R=haraken
BUG=420515

Review URL: https://codereview.chromium.org/1146903003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@195620 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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