Fix race in base::Timer when SetTaskRunner() is used

Fix race in base::Timer when SetTaskRunner() is used

Effectively reverting back to patch set 32 of
https://codereview.chromium.org/2491613004/ which had to land without
fixing the pre-existing race per the race proving harder than expected
to fix and not being worsened by that CL.

BUG=587199

[gab, 2017-05-26 05:26:32]

The CQ bit was checked by gab@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-05-26 05:27:05]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-05-26 06:31:36]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-05-26 06:31:36]

Dry run: Try jobs failed on following builders:
  linux_chromium_tsan_rel_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)