Cleanup transform-related methods in RenderLayer.

Cleanup transform-related methods in RenderLayer.

-Get rid of paintsWithTransform.
-Remove currentTransform. It was only ever called with
IncludeTransformOrigin and on non-null transforms.
-Remove renderableTransform. It was only called on
non-null transforms.
-Remove a layer()->parent() branch. The ASSERT had been
added in a previous patch, but the branch wasn't removed.
-Inline makeMatrixRenderable. It was only called in one
place.
-Add a test to make sure that transform origin is corrrectly
excluded from the transform computedStyle.

R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/4e0a4feff24e6e66c6efe075798bc611613fc4a0

[abarth-chromium, 2015-02-24 22:28:43]

abarth@chromium.org changed reviewers:
+ abarth@chromium.org

[abarth-chromium, 2015-02-24 22:28:44]

lgtm

[ojan, 2015-02-24 22:31:53]

Committed patchset #2 (id:20001) manually as
4e0a4feff24e6e66c6efe075798bc611613fc4a0 (presubmit successful).