Bug 860123 - Part 1: Disallow extending or collapsing selections across documents; r=ehsan

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extra : rebase_source : 2dbd8192486c8ff26d4b231d87283d61b6ff1e24
Bug 860123 - Part 2: Make mochitests expect the new behavior. r=ehsan

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extra : rebase_source : f73cceefbd3220d27b305c052125ebe5b2753c7e