Minor cleanups and improvements for the D-Bus library.

Minor cleanups and improvements for the D-Bus library.

- Add mock_export_object.{cc,h} to dbus.gyp, which were missing.
- Add a comment about shutdown of Bus in bus.h.
- Update mock_unittest.cc to call ShutdownAndBlock().
- Replace DCHECKs with LOG(ERROR)s followed by early exit.
- Add virtual to SetUp() and TearDown() in tests.
- Renamed a member variable to make it clearer.

BUG=chromium:90036
TEST=dbus_unittests


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=98560

[stevenjb, 2011-08-27 00:43:52]

LGTM

[commit-bot: I haz the power, 2011-08-27 07:26:44]

Change committed as 98560