perf: add a ShellGlobal property to call glFinish and log the time

It's useful to know how long frames are taking to render on the GPU.
This is impossible to measure in the normal case because frames may
parallelize with previous frames, but by calling glFinish() at the
end of the frame, we can create a (somewhat artificial, but useful)
environment where we have a meaningful timestamp for the frame
finishing drawing.

https://bugzilla.gnome.org/show_bug.cgi?id=732350
Add a new hwtest script

Add a performance test script that is used by the GNOME Hardware Testing
Project to measure metrics to be reported to perf.gnome.org.

https://bugzilla.gnome.org/show_bug.cgi?id=732350
Add --hwtest option to gnome-shell-perf-helper

Add a --hwtest option to gnome-shell-perf-helper which runs the
tests in perf/hwtest.js with the appropriate environment, and then
logs the results using the 'gnome-hwtest-log' utility which is
available in the hardware testing environent.

(For development of hwtest.js in a normal environment, run the tests
as: gnome-shell-perf-tool --perf=hwtest --extra-filter=Gedit)

https://bugzilla.gnome.org/show_bug.cgi?id=732350