Our own Inspector differs from most other DevTools implementations with
regard to highlighting DOM nodes as you hover elements in the inspected
DOM tree. In other implementations, as you change the hovered node, the
browser will render a box model overlay onto the page for that node. We
currently don't do this; we wait until you click the node, at which
point we both paint the overlay and inspect the node's properties.
This patch does not change that behavior, but separates the IPCs and
internal tracking of inspected nodes to support the standard DevTools
behavior. So the DOM document now stores an inspected node and a
highlighted node. The former is used for features such as "$0" in the
JavaScript console, and the latter is used for the box model overlay.
Our Inspector continues to set these to the same node.
We currently receive serialized JSON values over IPC and forward them to
them WebView callbacks, leaving it to the implementations of those
callbacks to parse the strings as JSON objects. This patch hoists that
parsing up to WebContentClient as soon as the IPC message is received.
This is to reduce the work needed for secondary implementations of these
callbacks (i.e. our Firefox DevTools server).
