Implement gecko leak checking for web-platform-tests, but do not
enable it yet due to oranges. Allows disabling leak checks for a
specific test using a 'leaks' key in test metadata. This causes the
browser to restart before running the (group of) tests with leak
checking disabled. The feature can be enabled by passing --leak-check
on the command line.
MozReview-Commit-ID: 1kBnJkOaeu8
1c362962c4