Add ASSERT to guard RenderLayer::location()

This CL adds an ASSERT to RenderLayer::location() to ensure that we don't read
this value between the time the RenderLayer is created and the time the value
is initialized.

I've also removed the setters for location and size. No one outside this class
should be setting these values directly. Unfortunately, there is one caller of
setSize in LayerTreeAsText, but that's called out as a hack in a comment. I've
renamed the setter to include the word "hack" to discourage future callers.

Review URL: https://codereview.chromium.org/423393009

git-svn-id: svn://svn.chromium.org/blink/trunk@179258 bbb929c8-8fbe-4397-9dbb-9b2b20218538