Bug 907118 - Fix bug in JSRope::getCharsNonDestructiveInternal (r=bhackett)

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extra : rebase_source : 30a52611309ba474653bd547cde3eb4af77dd578
Bug 908255 - tidy up 'non-destructive' functions, give StringHashPolicy a scarier name (r=terrence)

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extra : rebase_source : 84bbc6c0416190a9dae8c758262def530a591219