address review comments and tidy up

--HG--
rename : gfx/layers/client/ContentClient.cpp => gfx/layers/client/TiledContentClient.cpp
rename : gfx/layers/client/ContentClient.h => gfx/layers/client/TiledContentClient.h
rename : gfx/layers/composite/ContentHost.cpp => gfx/layers/composite/TiledContentHost.cpp
rename : gfx/layers/composite/ContentHost.h => gfx/layers/composite/TiledContentHost.h