Revert "[filesapp] Disable TabIndex/FilesAppBrowserTest fixtures"

This reverts commit d11568cadd35115a2b41d1590b1849c194fa8cbe.

Reason for revert: Change rolled into the LTS bot (see CL:2825639) so
revert this change to re-enable the TabIndex tests on ToT Chrome.

Original change's description:
> [filesapp] Disable TabIndex/FilesAppBrowserTest fixtures
>
> Files app TabIndex tests fail on the LTS bots at M86, M87. Disable the
> TabIndex tests on main (ToT Chrome) so we can later roll the change to
> the LTS bot branches.
>
> Bug: 1190837
> Change-Id: I3e08d7f2c4bfa8ca96f8226431cbfc75b8d69a1b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2825973
> Reviewed-by: Ben Pastene <bpastene@chromium.org>
> Commit-Queue: Noel Gordon <noel@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#872610}

Bug: 1190837
Change-Id: I031153d832e851a6c5153fcef47ed142efd2bcd7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2825640
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Noel Gordon <noel@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#872613}