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This reverts commit 413675e9.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 564933 as the
culprit for failures in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtLzQxMzY3NWU5YjY2N2UwN2FlODRhMTE1MjY4YWJlZGZkYjc0MzA3NDAM

Sample Failed Build: https://ci.chromium.org/buildbot/chromium.memory/Linux%20ChromiumOS%20MSan%20Tests/7480

Sample Failed Step: browser_tests

Original change's description:
> Run PrioritizedTaskRunner tasks with the same priority in posting order
> 
> What:
> If two tasks have the same priority, they should run in posting order.
> 
> A second thing this CL does is fix a race in the unittests. I meant to block
> the task runner before queueing up requests but failed to. Fixed here.
> 
> Why:
> This can prevent hypothetical task starvation. It's also necessary for the
> SimpleCachePrioritization control group to ensure that we maintain old
> behavior.
> 
> Bug: 850085
> Change-Id: I1f259296604bf04ea33ecbee9ad3180f064c4266
> Reviewed-on: https://chromium-review.googlesource.com/1088759
> Commit-Queue: Josh Karlin <jkarlin@chromium.org>
> Reviewed-by: Maks Orlovich <morlovich@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#564933}

Change-Id: Id4930090c0026aefa9d92bb8871e809a3fcfa124
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 850085
Reviewed-on: https://chromium-review.googlesource.com/1089892
Cr-Commit-Position: refs/heads/master@{#565096}