AW docs: document android_channel for AOSP integration

No change to logic. This adds 'android_channel' to the AOSP integration
doc, since the default no-channel build is typically meant for local
developers.

Fixed: 1085710
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: If45f36523a9b01315815257467bfd52c54bcf7eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2213926
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772233}

android_webview/docs/aosp-system-integration.md

@@ -195,6 +195,10 @@ is_component_build = false
 # Disable Google-specific branding/features
 is_chrome_branded = false
 use_official_google_api_keys = false
+
+# May disable some experimental (unstable) features. Hides WebView DevTools
+# (a debugging tool most users won't need to access).
+android_channel = "stable"
 ```
 
 The `target_cpu` option must be set to