Revert of Split preserve3d into: "should-flatten" and "is-3d-sorted" (https://codereview.chromium.org/147833003/)

Reason for revert:
Introduces flakiness in Blink layout tests:

http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%2540ToT%2520Blink&tests=compositing%252Fbackface-visibility-transformed

Original issue's description:
> Split preserve3d into: "should-flatten" and "is-3d-sorted"
> 
> Preserve3d has implications for both sorting and flattening of
> transforms. This is a bummer. Sometimes we want to allow a transform to
> remain unflattened, but we don't want that to affect 3d sorting. With
> this CL, we will have the ability to do that.
> 
> Note: this is essentially this cl
> https://codereview.chromium.org/100393005/
> 
> ..minus the generalizations to sorting that were being attempted in that
> patch.
> 
> Note, this CL is gated on https://codereview.chromium.org/98373011/ 
> 
> R=enne@chromium.org
> BUG=338980
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=249974

TBR=enne@chromium.org,shawnsingh@chromium.org,vollick@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=338980

Review URL: https://codereview.chromium.org/158563003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250031 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250031