Clarify when what notifications are send when a window object is cleared
    
There are two different events that most of the time coincide: a
JavaScript wrapper for a DOMWindow is created, and a new Document is
installed on the DOMWindow.
    
Sometimes, only the former happens, e.g. an iframe with an image as src
doesn't have an document.
    
However, for the embedder, this difference doesn't matter. It's in both
cases possible (and necessary) to install additional hooks on the
windows object.
    
Last but not least, we should never notify the embedder for changes in
isolated worlds. An embedder that is interested in these signals should
listen didCreateScriptContext/willReleaseScriptContext.
    
BUG=none
R=yurys@chromium.org,dcarney@chromium.org

Review URL: https://codereview.chromium.org/146693005

git-svn-id: svn://svn.chromium.org/blink/trunk@173621 bbb929c8-8fbe-4397-9dbb-9b2b20218538