Delete some more unused or singly-use FrameLoaderClient functions.

didFinishLoad() exists to call a plugin load observer that will never be there
for a same document navigation.

canShowMimeType() is called once in DocumentLoader, so make it a private static
in DocumentLoader.cpp

generatedMIMETypeForURLScheme() is never called.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/19696006

git-svn-id: svn://svn.chromium.org/blink/trunk@154498 bbb929c8-8fbe-4397-9dbb-9b2b20218538