Move ContentChangeType enum to a separate file, so that WebGLRenderingContextBase.h doesn't need LayoutBoxModelObject.h

BUG=614015
TBR=bajones@chromium.org
CQ_INCLUDE_TRYBOTS=tryserver.chromium.win:win_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2013603003
Cr-Commit-Position: refs/heads/master@{#395934}
3 files changed