browser.forkNewDriverInstance should return a promise that resolves to the new browser

[cnishina]

Reference #5019

protractor/lib/runner.ts

Line 275 in ed955e5

browser_.ready =

"I think this is so convoluted because it needed to support the control flow case as well. This should be cleaned up - forkNewDriverInstance() should just return a promise that resolves to the new browser instance, after its ready."

browser.forkNewDriverInstance should return a promise that resolves to the new browser.

[cnishina]

PR fix #5085