Bug 1103644 - Try default browser for "mach resource-usage" if the specified one doesn't work. r=gps DONTBUILD

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extra : amend_source : bd2dfce52c1647831dba1b3fbb2790bfce0beae0
extra : histedit_source : 2f0640e53e7ac359e4c050f73f86546518211e1e