This is just to help make the message handlers a bit briefer. I had
considered adding a TRY-like macro to auto-return when the lookup fails,
but since statement expressions cannot return references, that would
result in a copy of all e.g. object and array lookups.
The "from" field is required in every response. It is the name of the
actor sending the message. This patch fills in the "from" field in the
Actor base class so that subclasses don't have to.
There is a lot needed all at once to actually inspect a tab's DOM tree.
It begins with requesting a "watcher" from a TabActor. It seems there
can be many types of watchers, but here we implement the "frame" watcher
only. The watcher creates an "inspector", which in turn creates a
"walker", which is the actor ultimately responsible for serializing and
inspecting the DOM tree.
In between all that, the DevTools client will send a handful of other
informational requests. If we do not reply to these, the client will not
move forward with the walker. For example, the CSSPropertiesActor will
be asked for a list of all known CSS properties.
