Added a new layer impl type.
Added a new layer impl type.

We currently don't handle PaintedScrollbarLayerImpl which appears
in newly generated traces. This adds the handling to be the same as
other LayerImpls.

R=nduca@chromium.org
BUG=none

Review URL: https://codereview.appspot.com/13503043

git-svn-id: 3a56fcae908c7e16d23cb53443ea4795ac387cf2@942 0e6d7f2b-9903-5b78-7403-59d27f066143

bpsinc-native/src_third_party_trace-viewer,bpsinc-native/src_third_party_trace-viewer,bpsinc-native/src_third_party_trace-viewer,bpsinc-native/src_third_party_trace-viewer