This adds the ability to use the command line flag '--jscov-dir-prefix' to collect javascript code coverage from xpcshell tests and output it into the specified directory as a JSON file.
MozReview-Commit-ID: 3MZm73SNChL
--HG--
extra : transplant_source : c%9B%DE%A93w%E7%11%89%BE-%E8%D9%18%BC%12z%0A%0E%E4
A subsequent commit will change where the temporary directory is located
in certain test invocations. To do this, we need to teach the xpcshell
harness to use an alternate temporary directory instead of using the system
default (likely specified from TEMP* environment variables).
MozReview-Commit-ID: IUUlYaLBiEC
--HG--
extra : rebase_source : 389144dedb8cce62c1d10c65a37be7fb632c2317
