Add a unit test for installing an extension with a 2048-bit key

BUG=293661

Review URL: https://chromiumcodereview.appspot.com/23531061

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224925 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224925