dbus: Property<>.value() should be const
dbus: Property<>.value() should be const

Constant references to PropertySets are a useful way to avoid someone
calling Get() and Set(), but we should allow them to call value().

BUG=none
TEST=built and tested

Change-Id: Ic8e5449e87ba3cebbe6a3534596bb53016236333


Review URL: http://codereview.chromium.org/9414029

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@122467 0039d316-1c4b-4281-b951-d872f2087c98

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