<webview>: Resolve relative paths as chrome-extension: URLs

The CL performs URL resolution in the browser process out of convenience. If this was done in the embedder process in web_view.js then URL resolution would need to be done on both attribute mutation and initialization. BrowserPluginGuest::OnNavigateGuest gets called either way.

When updating the src attribute on loadcommit, we can no longer do a simple string comparison, as the resolved URL may not match the developer-assigned URL.

BUG=308114
Test=<webview src='guest.html'></webview> will attempt to resolve to
chrome-extension://appID/guest.html.

R=creis@chromium.org

Review URL: https://codereview.chromium.org/27408004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232203 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232203