We use _pretty_path when specifying the targets of generated files, so
we need to use _pretty_path for the inputs as well. Otherwise make won't
know that they refer to the same file, and result in "No rule to make
target" errors.
MozReview-Commit-ID: JTdLFbkX1J0
The css_properties.js rule can be converted into gen-css-properties.py,
which we can install with TEST_HARNESS_FILES instead of the
mochitest.ini manifest.
MozReview-Commit-ID: F7nf71ORWsS