Clarify WebScrollbarThemeGeometry ownership transfer in createScrollbarLayer().

This CL adds a commment for clarifying ownership transfer.

Review URL: https://codereview.chromium.org/26716002

git-svn-id: svn://svn.chromium.org/blink/trunk@159300 bbb929c8-8fbe-4397-9dbb-9b2b20218538