Reland "Refactor wpt-import to use common.checkout.git"

This is a reland of cd0c516c9dbe22b47d7f4325baae0a3b411ed634

Compared with the original CL, this one instantiates self.wpt_git
after local_wpt.fetch() to ensure local_wpt.path exists so that
Git.__init__ does not throw.

Original change's description:
> Refactor wpt-import to use common.checkout.git
>
> Some methods are not currently supported by common.checkout.git, and
> Git.run is used in such cases for now. Later we would like to port some
> of them (the reusable ones) to common.checkout.git.
>
> Bug: 676399
> Change-Id: I42459221c684f8ffeabb9991a64dea3489663d19
> Reviewed-on: https://chromium-review.googlesource.com/793984
> Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
> Commit-Queue: Robert Ma <robertma@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#519777}

Bug: 676399
Change-Id: I919064f5599a1b503a9ce5db927cc6713d9f428f
Reviewed-on: https://chromium-review.googlesource.com/794610
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520115}
2 files changed