Bug 829439 (part 1) - Add MemoryReporterBase class that promotes better encapsulation within nsIMemoryReporter sub-classes.  r=jlebar.

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extra : rebase_source : 53a77ea456f2aedafe05954ceece7e64db743e66
Bug 829439 (part 2) - Some memory reporter clean-ups.  r=jlebar.

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extra : rebase_source : 31ef4aaa1dde5299125ed67d297d70ee5fe5c876