In case of multiple sessions to the same target, there's a race between
sessions to create a secondary isolated world. As a result, we might
end up having 2 execution contexts created for the needs of the
secondary isolated world.
This patch starts handling this race gracefully: instead of crashing,
we can use either of the execution contexts and ignore the rest.
Notably, the same race condition might happen if page reloads itself
in-between the calls to `page.addEvaluateOnNewDocument` and
`page.createIsolatedWorld`.
Fixes#4197.
Method `page.setDefaultTimeout` overrides default 30 seconds timeout
for all `page.waitFor*` methods, including navigation and waiting
for selectors.
Fix#3319.
`page.waitForSelector` should return `null` if waiting for `hidden:
true` and there's no matching node in DOM.
Before this patch, `page.waitForSelector` would return some JSHandle
pointing to boolean value.
This patch splits out `IsolatedWorld` class from Frame.
The `IsolatedWorld` abstraction is an execution context
with a designated set of DOM wrappers.
References #2671
