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This reverts commit bb8a36eb.

Reason for revert: Created too much load on too-small Xenial
pool, resulting in large pending test shard times, resulting
in long builds, resulting in pending times, resulting in even
longer builds due to cold caches, resulting in even longer
pending times.

Original change's description:
> [testing] switch to use xenial for swarming test
> 
> This CL switches to use xenial for test as many as possible.
> 
> Sheriff:
> If this broke some ci/fyi builder, feel free to revert this.
> 
> TBR=jbudorick
> 
> Bug: 954450
> Change-Id: I98abfb50bb10ca3eeec0e23d197fe7672c8acc95
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1689513
> Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
> Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#675527}

TBR=yyanagisawa@chromium.org,tikuta@chromium.org,jbudorick@chromium.org

Change-Id: I1244abdfa1cf627c7f9effa3048ccf8ed53091b8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 954450, 982335
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692035Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675722}