Move isLoading() from WebFrame to WebLocalFrame

It is a concept that doesn't make sense for a remote frame (whether the load
event has fired), and all callsites are using WebLocalFrames

BUG=

Review URL: https://codereview.chromium.org/555953002

git-svn-id: svn://svn.chromium.org/blink/trunk@181689 bbb929c8-8fbe-4397-9dbb-9b2b20218538