Add missing R class to WebView resource rewriting.

Add missing R class to WebView resource rewriting.

We weren't rewriting the resources for web_contents_delegate_android,
resulting in crashes any time a resource was loaded by that component
(e.g. popup bubbles for HTML form validation failures). Add it to the
list and also clean up outdated comments here that refer to previous
versions of this code. Add a TODO to track finding a better way to do
this which is less fragile.

BUG=490826

Review URL: https://codereview.chromium.org/1154853005

Cr-Commit-Position: refs/heads/master@{#331356}