Prevent extensions from defining homepages with schemes other than valid web extents.

      BUG=84402
TEST=ExtensionManifestTest.ParseHomepageURLs
Review URL: http://codereview.chromium.org/7089014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@87722 0039d316-1c4b-4281-b951-d872f2087c98