MV3: Request content-script config on first tick

[sammacbeth]

The navigator-interface test is often flakey on MV3. This seems to be because, on a slow machine, extension startup and content-script registration are slow. The means that we actually load the test page before the content-scripts are registered.

Now that we exposed MV3 content-script registration via a component, we can wait for that to complete before loading the test page.

[sammacbeth]

It seems this doesn't fully fix it - it looks like in some cases we're just too slow to get the config to C-S-S.

In the MV3 content-script, we're waiting for the secret from the page script before we request the config. We can do these things in parallel to reduce the overall latency to deliver the config to the page script.