There should only be one isMainFrame()

Instead of having a bunch of wrappers lets just call isMainFrame(). It's
a bit more verbose in some places, but it makes the code more obvious
too. Especially since it was really weird that RenderLayerCompositor
would have a method like isMainFrame() since it's not a Frame at all.

I also removed a null check in RenderView for the frame. If you have a
RenderView, you have a frame.

Review URL: https://codereview.chromium.org/238763002

git-svn-id: svn://svn.chromium.org/blink/trunk@171557 bbb929c8-8fbe-4397-9dbb-9b2b20218538