Remove more redundant code from CompositedLayerMapping.

Its amazing what a little use of helper functions
can do for code readability.

Should be no functional change.

BUG=338896

Review URL: https://codereview.chromium.org/149413007

git-svn-id: svn://svn.chromium.org/blink/trunk@166083 bbb929c8-8fbe-4397-9dbb-9b2b20218538