fix(Page): Page.waitForNavigation should correctly handle mixed content (#2339)

This patch teaches Page.waitForNavigation to correctly handle navigation
to pages that have frames that might never load.

These frames include:
- frames which main resource loading was aborted due to mixed-content
  error
- frames that artificially called `window.stop()` to interrupt loading
  themselves

Fixes #1936.

test/assets/frames/one-frame.html

@@ -0,0 +1 @@
+<iframe src='./frame.html'></iframe>