Revert "Remove headless_browsertests from Fuchsia bots"

This reverts commit 11b7e1fa.

Reason for revert: Package loading issue has been fixed upstream.

Original change's description:
> Remove headless_browsertests from Fuchsia bots
>
> Bug: 1198259
> Change-Id: I2cefa5f6afb77a3e2e84275112e89137bd28df41
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2822346


> Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
> Commit-Queue: Dirk Pranke <dpranke@google.com>
> Auto-Submit: Andrey Kosyakov <caseq@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Cr-Commit-Position: refs/heads/master@{#871616}

Bug: 1198259, 1198143
Change-Id: I946419447ad99350df5819419510cf503caab8bb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2821519


Auto-Submit: Wez <wez@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#871961}