Bug 889129 - Fix performance on Splay with generational GC enabled; r=bhackett

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extra : rebase_source : bebc97112c6d7e09d42d7083e49b7bb6cd6dceae
Bug 913260 (part 1) - Simplify TelemetryPing.js a little.  r=nfroyd.

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extra : rebase_source : 0c7e18e269a49ca6dc79623ebc370a6c8440a663