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/commands/schema_utils.h b/buffet/commands/schema_utils.h
index a05d080..33261f0 100644
--- a/buffet/commands/schema_utils.h
+++ b/buffet/commands/schema_utils.h
@@ -120,10 +120,10 @@
 
 // Converts PropValue to Any in a format understood by D-Bus data serialization.
 // Has special handling for Object types where native_types::Object are
-// converted to chromeos::dbus_utils::Dictionary.
+// converted to chromeos::VariantDictionary.
 chromeos::Any PropValueToDBusVariant(const PropValue* value);
 // Converts D-Bus variant to PropValue.
-// Has special handling for Object types where chromeos::dbus_utils::Dictionary
+// Has special handling for Object types where chromeos::VariantDictionary
 // is converted to native_types::Object.
 std::shared_ptr<const PropValue> PropValueFromDBusVariant(
     const PropType* type,