Bug 1061024 (part 1) - Don't run DMD when about:memory's "Measure" button is pressed. r=jld.

--HG--
extra : rebase_source : ddf0236352c64eb9839dfd0a219451a568aba10e
Bug 1061024 (part 2) - Some minor DMD clean-ups. r=jld.

--HG--
extra : rebase_source : f780376e8803222c92a5a92bc4155600e068ce0c
Bug 1061024 (part 3) - Remove a redundant dmd::ClearReports() call. r=jld.

--HG--
extra : rebase_source : 5c79c9187fbffe2b5a8c63184cc9d3f158877a8e