This is a reland of c40a21f3

Changes: Fixed presubmit failing on Windows due to being unable

to find gn, fixed export_targets.py failing on Windows for

Googlers due to being unable to find Visual Studio files.

Original change's description:
> Add export_targets.py to presubmit
>
> Adds export_targets.py to run as part of presubmit in order to help
> prevent breaking Firefox with BUILD.gn changes.
>
> Bug: chromium:1003151
> Change-Id: I5a7ab00891cd7c094c797e6150f642f803a726b6
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1802038
> Commit-Queue: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>

Bug: chromium:1003151
Change-Id: I321ade86f2d969601afb8e1ed61c36bf166887b5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1842127
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>