Auto merge of #15379 - indygreg:mozilla-central-embedding, r=emilio

Find WPT files in Firefox repository

The Servo repository is now (mostly) vendored in the Firefox Mercurial
repository. For size and duplication reasons, the tests/wpt directory is
not included in the vendored copy.

This causes problems when running `mach` from the Firefox repository
because `mach` references pip requirements files and module search
paths from WPT.

This commit adds code to detect when Servo's mach is running from a
Firefox source tree and to resolve WPT paths to the Firefox location
if appropriate. This enables `mach` to "just work" when running
from the servo/ directory in the Firefox repository.

The file looked for to identify the Firefox repository is identical
to what Firefox's `mach` script uses.

A potential issue with using Firefox's WPT files is that they may be
different from those in the Servo repository and this could lead to
differences in behavior - possibly even an error when loading/running
`mach`. However, the behavior before this commit was that Servo's
`mach` never worked in the Firefox repository (due to missing WPT
files). And post-commit it does. So this seems like a "perfect is the
enemy of good" scenario.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/15379)
<!-- Reviewable:end -->

0 files changed, 0 insertions, 0 deletions