This is a reland of fc9a40e3

Hopefully the cause of the 2nd revert was fixed in https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3489327

Original change's description:
> Reland "Add support for Gerrit topics in gclient syncs"
>
> This is a reland of 0f13273f
>
> Hopefully the cause of the original revert was fixed in https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3480835
>
> Original change's description:
> > Add support for Gerrit topics in gclient syncs
> >
> >
> > If the new flag "--download-topics" is specified with a "--patch-ref" then:
> > * Finds the topic of the Gerrit change.
> > * Finds all open changes in the same repo as the Gerrit change.
> > * Cherrypicks all changes locally.
> >
> > This functionality can be used by developers and bots to apply all changes with the same topic in the checkout to be tested at the same time (similar to how Android's TreeHugger handles topics).
> >
> >
> > Tested by:
> >
> > * Running the new unit test with `python gclient_scm_test.py GerritChangesTest.testDownloadsTopics` from the `tests/` directory.
> >
> > * Running an end-to-end test with `DEPOT_TOOLS_UPDATE=0 gclient sync --patch-ref "skia@d831da5b8ac2d257c5b0cf2ec6645a148f05e662:refs/changes/17/505217/2" --download-topics` in a skia checkout.
> >
> >
> > Bug: chromium:1298922
> > Change-Id: Ieace5e27fbc9c5d0ea90a037bf80a95062c1b164
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3444003
> > Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> > Commit-Queue: Ravi Mistry <rmistry@chromium.org>
>
> Bug: chromium:1298922
> Change-Id: I80747d797234bba06c17ef5c5e85b310281922c4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3484976
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Commit-Queue: Ravi Mistry <rmistry@chromium.org>

Bug: chromium:1298922
Change-Id: I21d7251bafff808b1144d6e522fa9f384f4541bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3490488Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Ravi Mistry <rmistry@chromium.org>