Encapsulate FontFallbackList's page access logic.

Minor cleanup: add a getter & setter to avoid code duplication in
Font.cpp.

R=jbroman@chormium.org,eae@chromium.org

Review URL: https://codereview.chromium.org/362963003

git-svn-id: svn://svn.chromium.org/blink/trunk@177397 bbb929c8-8fbe-4397-9dbb-9b2b20218538