This roll moves Juggler component registration to a new system that
is used in XPCOM: this is what both RemoteAgent and Marionette use.
Drive-by: we now have to explicitly pass default values to dispatched
mouse events to avoid a `MOZ_ASSERT` trigger inside a drag service
native code.
Manual conflict resolutions: ab3ed0cb2c
* `clip` option is always passed from the client code
* with this change, we can no longer capture screenshot of a blinking
caret; the browser-side API doesn't have this capability.
