This moves setting the navigator.webdriver flag from after WebContent
creates the WebDriver connection, to its own IPC to be triggered from
WebDriver. This is closer to the spec, but mostly serves as an easy
test to validate the connection.
This just sets up the infrastructure for the WebContent process to house
WebDriver IPCs, and adds an IPC for WebContent to create the WebDriver
connection. The WebDriverConnection class inside WebContent ultimately
will contain most of what is currently in WebDriver::Session (so the
copyright attributions are copied here as well).
The socket created by WebDriver is currently /tmp/browser_webdriver
(formatted with some IDs). This will be moved to the /tmp/webdriver
folder, as WebDriver will create multiple sockets to communicate with
both Browser and WebContent as the IPCs are iteratively moved to
WebContent. That path is unveiled here, though it is unused as of this
commit.
