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>

buffet/commands/dbus_command_proxy.cc

diff --git a/buffet/commands/dbus_command_proxy.cc b/buffet/commands/dbus_command_proxy.cc
index 4bbf1ab..0b97c06 100644
--- a/buffet/commands/dbus_command_proxy.cc
+++ b/buffet/commands/dbus_command_proxy.cc
@@ -61,7 +61,7 @@
   progress_.SetValue(command_instance_->GetProgress());
   // Convert a string-to-PropValue map into a string-to-Any map which can be
   // sent over D-Bus.
-  chromeos::dbus_utils::Dictionary params;
+  chromeos::VariantDictionary params;
   for (const auto& param_pair : command_instance_->GetParameters()) {
     params.insert(std::make_pair(param_pair.first,
                                  param_pair.second->GetValueAsAny()));