This adds support for viewing request payloads (POST data) and response
bodies in the Firefox DevTools network panel.
Request bodies are captured when network requests start and passed
through IPC to the NetworkEventActor, which returns them via the
getRequestPostData protocol method.
Response bodies are streamed via a new IPC message as data is received,
accumulated in NetworkEventActor (with a 10MB size limit to prevent
memory issues), and returned via getResponseContent. Text content is
returned as UTF-8, while binary content (images, etc.) is base64.
Hook ResourceLoader to emit network request lifecycle events through
IPC to the UI process, where FrameActor creates NetworkEventActor
instances that serialize requests using Firefox's Remote Debug Protocol.
The Network panel now shows requests with method, URL, status, MIME
type, size, and timing information. Several features remain stubbed
(POST data, response content, cause detection) marked with FIXMEs.
