chromeos-dbus-bindings: Create proxy generator Consolidate common functions to a parent HeaderGenerator class, and create a new ProxyGenerator class. This new class converts from native C++ method calls to D-Bus method invocations. It also takes an interface class (populated with default null implementations) for all signals supported by the interface. CQ-DEPEND=CL:221015 BUG=chromium:404505 TEST=New unit test Change-Id: Id6ed1f6f6e0868f32911a96e23e3dc74c9df0212 Reviewed-on: https://chromium-review.googlesource.com/221365 Reviewed-by: Alex Vakulenko <[email protected]> Commit-Queue: Paul Stewart <[email protected]> Tested-by: Paul Stewart <[email protected]>