Pass 3d rendering context / should-flatten info from blink to cc

The compositor should not know about preserve-3d; it's a blink concept. Instead,
it should operate in terms of rendering-context (to figure out which layers sort
with respect to each other) and should-flatten (for flattening, obviously).

This change prepares blink to provide this information to cc. In a later CL,
cc will start taking advantage of this information.

Note: this CL is a prereq for https://codereview.chromium.org/147833003/

R=shawnsingh@chromium.org
BUG=322216, 338980

Review URL: https://codereview.chromium.org/98373011

git-svn-id: svn://svn.chromium.org/blink/trunk@166745 bbb929c8-8fbe-4397-9dbb-9b2b20218538