Rename constant to kCubeFaceCount. This corrects the naming style for this constant variable. Also updates many usages in TextureD3D.cpp and TextureGL.cpp. Bug: angleproject:2470 Change-Id: I029dd98e925cabf23a4bb26467a62afe6c5bb6e0 Reviewed-on: https://chromium-review.googlesource.com/1220726 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>