It seems that running the PGO step multiple times isn't having the effect I expected. This probably isn't the reason for the lack of success, but it's simple enough to clear the profile each time we run the instrumented build. Patch in a moment.
Created attachment 544019 [details] [diff] [review] Construct a new profile each time we run an instrumented build during PGO.
In build-system http://hg.mozilla.org/projects/build-system/rev/24e8f700dfbf