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This reverts commit 1d114582.

Reason for revert: it is breaking the Android tests https://ci.chromium.org/p/chromium/builders/ci/Android%20FYI%20Release%20%28Nexus%209%29/27588

Original change's description:
> Fix sequence checker asserts in CrxDownloader.
> 
> The code crashes because there is a race condition between
> the main sequence and another task runner when the instances
> of the CrxDownloader are destroyed.
> 
> On the main sequence, the CrxDownloader is being used to
> download a file, then the reference to this instance
> is released. The CrxDownloader is using a task runner
> to run a blocking task, which validate the file it has just
> downloaded. This task runner receives a closure which holds a
> reference to the CrxDownloader.
> The reference is released after the task is run, and therefore,
> it results in a race when the dtor of CrxDownloader is being
> invoked.
> 
> Besides removing the offending sequence checkers of the
> destructors of the CrxDownloader classes, this CL changes
> where the timer of the BackgroundDownloader is destroyed.
> The timer has sequence affinity and it must always
> be destroyed on the main sequence.
> 
> Bug: 1105289,1105460
> Change-Id: I78e002502ce6f5501c78d1e849f51b47bf3dc092
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2297696
> Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
> Commit-Queue: Sorin Jianu <sorin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#788330}

TBR=sorin@chromium.org,waffles@chromium.org

Change-Id: I87c30dbc1542c14e18c69c0806f138508f99bf9a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1105289, 1105460
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2298439Reviewed-by: Sorin Jianu <sorin@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788373}