Add system util to execute app and retrieve its output This will be useful to run external applications, such as benchmarks, and process their output. Bug: angleproject:3125 Change-Id: Ic13c69f2e034f4b47498fb2f299c62423c355c4a Reviewed-on: https://chromium-review.googlesource.com/c/1452534 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@google.com>