feat: migrate protocol module to NetworkService (Part 9) (#18374)

* Compare final data instead of url

The behavior of did-finish-load and getURL has changed for redirects when
using NetworkService, so the test fails for NetworkService.

Comparing the finally received data makes the test more reliable.

* Implement intercept APIs

* Setting mimeType should set "content-type" header

* Passing no argument should not throw JS error

* Don't access api namespace in ProxyingURLLoaderFactory

* No need to create AtomURLLoaderFactory every time

* No use of weak factory

spec/api-protocol-spec.js

@@ -310,8 +310,8 @@ describe('protocol module', () => {
       const handler = (request, callback) => callback({ url: redirectURL })
       await registerHttpProtocol(protocolName, handler)
 
-      await contents.loadURL(url)
-      expect(contents.getURL()).to.equal(url)
+      const r = await ajax(url)
+      expect(r.data).to.equal(text)
     })
 
     it('can access request headers', (done) => {