This patch creates the first full mochitest that exercises the profiler
popup's mechanisms of capturing profiles. The test tries to use user-focused
mechanisms–clicking buttons to fully capture a profile.
In addition, it fixes two leaks that were uncovered by the leaktest check.
The first has to do with the iframe src creating an about:blank page when
set to an empty string. The next was the Services.obs.addObserver call
in the perfFrontInterface not being removed when the page was unloaded.
Differential Revision: https://phabricator.services.mozilla.com/D45530
f5e92e8381