Cleanup directory structure

The src/core/renderer directory is meant to contain files that would be
in the renderer process and the chrome/renderer subdirectory in chromium,
but a few classes from the browser process and the renderer_host dir
has snuck in.

This patch cleans up the structure so that classes in the browser that
serves as host classes for renderer classes goes in renderer_host.

Change-Id: I9333b1322e2246d4da9b4e8cfe6be604f6d996bf
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>