Bug 927740 (part 1) - Fix observation of GC and CC requests by child processes.  r=khuey.

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extra : rebase_source : 97e44a40d98ad71152dd24a8ea6ff0098aab1df4
Bug 927740 (part 2) - Make about:memory's "Minimize Memory Usage" button affect child processes, like the "GC" and "CC" buttons.  r=khuey.

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extra : rebase_source : 555f0e6ca11caa73e33f48a941ee119181c28b74
Bug 927740 (part 3) - Make workers respond to "child-cc-request" notifications.  r=khuey.

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extra : rebase_source : 34a15c4bf15bb0fd1601675e7e7cc4b5117ed433
Bug 927740 (part 4) - Make workers CC on memory-pressure.  r=khuey.

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extra : rebase_source : 372b3de683342634a182d3e782016718c067cead