Don't implicitly force use of WebDriver's ManagedPromise

[sjelin]

There are a couple different places, scattered throughout out codebase, where we force ManagedPromise accidentally. As far as I've noticed, this happens in one of the following ways:

• We use wdpromise.fulfilled, which is actually deprecated. Luckily, we can just switch to WebDriver.promise.when.
• We use wdpromise.defer. We can get around this with new wdpromise.controlFlow().promise(...), which follows the same syntax as for ES6 promises. (We only use wdpromise.defer in the debugger, which has a separate issue: Debugger needs to work with the WebDriver control flow turned off #3901)
• (I may add to this list if I find more) (I did not)

[sjelin]

After #4017 gets merged in, the only instances of wdpromise.defer will be in lib/debugger.ts. I'm making a note in the corresponding issue (#3901), and I'm limiting the scope of this issue to wdpromise.fulfilled