Fix appViewGuestInternal error in background pages.

Fix appViewGuestInternal error in background pages.

Make appViewGuestInternal available in all channels, note that this
doesn't expose <appview> to all channels, appViewEmbedderInternal
controls that.
Making appViewGuestInternal available in all channel would make
require('binding').Binding.create('appViewGuestInternal').generate();
from app_runtime_custom_bindings.js not complain.

BUG=437891
Test=Note that this fix is only going to show up in stable/beta.
1) Get unpacked calculator app from here:
https://github.com/GoogleChrome/chrome-app-samples/tree/master/samples/calculator
2) This app has background page.
Open the app in chrome stable or beta.
3) Right click and select "Inspect background page".
The following error in background page's console should not
show up.
"chrome.appViewInternal is not available: 'appViewInternal' requires Google Chrome dev channel or newer, but this is the stable channel."

Review URL: https://codereview.chromium.org/773583003

Cr-Commit-Position: refs/heads/master@{#306310}