Auto merge of #13370 - heycam:gecko-arc, r=Manishearth

Rename GeckoArc macro so it sounds like it's useful for already-threadsafely-refcounted objects.

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This is just some renaming.  I will want to use this macro with some objects that support thread-safe refcounting, and which thus don't use `nsMainThreadPtrHolder`.  For fun I pushed a stylo try run with this change: https://treeherder.mozilla.org/#/jobs?repo=try&revision=4b6d906b423e

r? @Manishearth

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