Remove unneeded break in CompositedLayerMapping::containingSquashedLayer

Remove unneeded break because the return makes it unreachable.

Review URL: https://codereview.chromium.org/392403003

git-svn-id: svn://svn.chromium.org/blink/trunk@178357 bbb929c8-8fbe-4397-9dbb-9b2b20218538