Disallow navigation to documents not explicitly listed as web accessible.

      The existing check for web accessible resources is inadequate and allows
navigations to non-whitelisted pages to succeed. This patch ensures that
the document is listed explicitly in the manifest when a navigation is
performed to it.

BUG=576867

Review-Url: https://codereview.chromium.org/2007133004
Cr-Commit-Position: refs/heads/master@{#397066}