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
