Remove redundant code for setting perspective transform

perspectiveTransform() already returns the identity matrix
when there is no perspective, so no need to avoid calling it.
Also, as far as I can tell clearing the transform on the graphics
layer is always OK when setting the clipLayer transform.

BUG=338896

Review URL: https://codereview.chromium.org/131003008

git-svn-id: svn://svn.chromium.org/blink/trunk@166011 bbb929c8-8fbe-4397-9dbb-9b2b20218538