Bug 1419666 - MemoryBlockCacheTelemetry::NotifyCombinedSizeGrown() should assert the main thread only for the first call. r=bechen,gerald

For the 2nd call might happen off the main thread.

MozReview-Commit-ID: 701p2GGuiEo

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extra : rebase_source : 049d91e3ecc86d3776ad6809e5e22678bd713a27