Bug 845609 (part 1) - Add a script to diff JSON memory report dumps.  r=kats.

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extra : rebase_source : 2c549d1f9f495811e0d0ecad9210145747a0725d
Bug 845609 (part 2) - Add a diff mode to about:memory.  r=jlebar.

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extra : rebase_source : 2819dd753dd7804c40ca20ac5fcee14e822307a0