libchromeos: Move Dictionary class into its own header file

Moved chromeos::dbus_utils::Dictionary type alias into its
own header file (chromeos/dictionary.h) and promoting it to the
top level namespace (chromeos).

So now it becomes chromeos::VariantDictionary and it is now
independent from D-Bus data serialization.

BUG=None
TEST=FEATURE=test emerge-link libchromeos buffet

Change-Id: Ibad63fd4168d77b9a4b80c7800016510f87ae33d
Reviewed-on: https://chromium-review.googlesource.com/219413
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/manager.cc b/buffet/manager.cc
index 03a5443..c39ec52 100644
--- a/buffet/manager.cc
+++ b/buffet/manager.cc
@@ -118,7 +118,7 @@
 
 void Manager::HandleUpdateState(
     chromeos::ErrorPtr* error,
-    const chromeos::dbus_utils::Dictionary& property_set) {
+    const chromeos::VariantDictionary& property_set) {
   state_manager_->UpdateProperties(property_set, error);
 }