Bug 964238 (part 1) - Remove JSStableString and StableTwoByteChars. r=terrence.

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extra : rebase_source : 1859ce0b42661494fa13628551359a5a5d5663c0
Bug 964238 (part 2) - Remove AutoStringRooter, because it's no longer used. r=terrence.

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extra : rebase_source : 3d3253f2bc1a867679841d60e4b49c62b8f917c5