This patch starts executing frame.waitForSelector and frame.waitForXPath
in secondary world. As a result, websites that mutate page global
context (e.g. removing global MutationObserver) don't break Puppeteer's
behavior.
Fixes#609
`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.
