Roll Fuchsia SDK from 6.20210906.2.1 to 6.20210906.3.1

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/fuchsia-sdk-chromium-autoroll
Please CC chrome-fuchsia-gardener@grotations.appspotmail.com on the revert to ensure that a human
is aware of the problem.

To file a bug in Chromium: https://bugs.chromium.org/p/chromium/issues/entry

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

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Tbr: chrome-fuchsia-gardener@grotations.appspotmail.com
Disable-Retries: true
Change-Id: I0b4ca000e7d4c08bd9b686e3f191dd36358edc58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3143368
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Cr-Commit-Position: refs/heads/main@{#918708}