Revert of Disallow posting tasks to SequencedWorkerPools by default. (patchset #24 id:460001 of https://codereview.chromium.org/2445763002/ )

Reason for revert:
Probable cause of test failures: "cronet_test_instrumentation_apk" on "Android Cronet Builder (dbg)"

https://build.chromium.org/p/chromium.android/builders/Android%20Cronet%20Builder%20%28dbg%29/builds/3581

Original issue's description:
> Disallow posting tasks to SequencedWorkerPools by default.
>
> We have observed crash reports in which SequencedWorkerPool workers
> were active while redirection to TaskScheduler was enabled. This can
> happen if a task is posted to a SequencedWorkerPool before redirection
> to TaskScheduler is enabled. With this CL, DumpWithoutCrashing is
> called if a task is posted to a SequencedWorkerPool too early. This
> will help us find offending call sites.
>
> BUG=622400
>
> Committed: https://crrev.com/4606ada249a240e115a08a7c66409e8fdd457ea1
> Cr-Commit-Position: refs/heads/master@{#432913}

TBR=gab@chromium.org,brettw@chromium.org,fdoray@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=622400

Review-Url: https://codereview.chromium.org/2517443002
Cr-Commit-Position: refs/heads/master@{#432989}
11 files changed