buffet: Add DBus proxy class for command instance object

Added DBusCommandProxy class that implements
org.chromium.Buffet.Command DBus interface, including command
methods and properties.

BUG=chromium:374864
TEST=USE=buffet P2_TEST_FILTER="buffet::*" FEATURES=test emerge-link platform2

Change-Id: Iaf17f2b7c276edc1e9f3ca09a759a4a7d4dc3b10
Reviewed-on: https://chromium-review.googlesource.com/213267
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>

buffet/dbus_constants.cc

diff --git a/buffet/dbus_constants.cc b/buffet/dbus_constants.cc
index d8c70ba..7fc0acf 100644
--- a/buffet/dbus_constants.cc
+++ b/buffet/dbus_constants.cc
@@ -22,6 +22,29 @@
 const char kManagerUpdateStateMethod[]      = "UpdateState";
 const char kManagerTestMethod[]             = "TestMethod";
 
+const char kCommandInterface[] = "org.chromium.Buffet.Command";
+const char kCommandServicePathPrefix[] = "/org/chromium/Buffet/commands/";
+
+const char kCommandSetProgress[] = "SetProgress";
+const char kCommandAbort[] = "Abort";
+const char kCommandCancel[] = "Cancel";
+const char kCommandDone[] = "Done";
+
+const char kCommandName[] = "Name";
+const char kCommandCategory[] = "Category";
+const char kCommandId[] = "Id";
+const char kCommandStatus[] = "Status";
+const char kCommandProgress[] = "Progress";
+
+const char kCommandStatusQueued[] = "queued";
+const char kCommandStatusInProgress[] = "inProgress";
+const char kCommandStatusPaused[] = "paused";
+const char kCommandStatusError[] = "error";
+const char kCommandStatusDone[] = "done";
+const char kCommandStatusCanceled[] = "canceled";
+const char kCommandStatusAborted[] = "aborted";
+const char kCommandStatusExpired[] = "expired";
+
 }  // namespace dbus_constants
 
 }  // namespace buffet