Add some test cases involving Symbol.iterator as a prelude to bug 918828. no_r=me, testonly.

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extra : rebase_source : 1bd8721f904456a7805d75e6e6b91c6cdf9eabb1
Bug 1082672, part 1 - Add JSAPI macros JS_SYM_FN etc. to support defining functions with well-known symbol keys. r=Waldo.

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extra : rebase_source : 9791c940599844802c9a262fe8b1610a0de3ef40
Bug 1082672, part 2 - Change mozilla::dom::GetArrayIndexFromId to cope with symbols. r=bz.

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extra : rebase_source : f183b8c543e42664e2374954b627600a5de5a42d
Bug 1082672, part 3 - Add some more symbol support for DOM bindings. r=bz.

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extra : rebase_source : 79b314cb464e9c77ac25bc19e91b6e13aa241aeb
Bug 1082672, part 4 - Change XrayWrapper code to be able to resolve symbol-keyed methods. r=bz, r=bholley.

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extra : rebase_source : f78cbb83f63dfffd648c6d3c280273f4a61c9fe8
extra : amend_source : f006a096174eee166125430753e65e9a31bd930b