Upstream: Launch WebAPK without showing intent picker when user taps link in WebAPK scope

This CL surpresses the intent picker when:
- User navigates via a link
AND
- The URL falls in the scope of a WebAPK
AND
- The WebAPK is the only specialized handler for the URL

BUG=609122
TEST=ExternalNavigationHandlerTest.*

Review-Url: https://codereview.chromium.org/2035183002
Cr-Commit-Position: refs/heads/master@{#398946}
7 files changed