Ignore frames in chrome.app.window.getAll() if the context is uninitialized.

This CL fixes an issue where calling getAll() would somehow break
extension renderers and cause them to not get their extension bindings.

Although the exact mechanism of failure is unknown, windows shouldn't be
accessible until their contexts are initialized anyway.

This CL speculatively fixes a Citrix issue. This CL is being landed so
that they can test on Canary. This CL will be rolled back before branch
point. See bug for more details.

Bug: 1021014
Change-Id: Ia32570b4eb4168f32a41ada60f8d17fe6104f81e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2991424
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#899450}