Previously, a race condition could cause DevTools to enter a state where it can't detect an application on reload. This was caused by a sequencing issue between the content script connection, the devtools panel connection and an event "backendReady" that lets DevTools know when a particular frame is ready to be inspected.
This commit replaces the previously stored backendReady boolean with a promise, so that the devtools panel can eventually run a callback to connect to a content script when that content script emits it's backendReady message.
PR Close#54805
