Remove indirection plumbing in CSSFontSelector.

Now that fontFaceCache is a direct accessor, we no longer
need wrappers around adding/removing items to/from it.

R=ksakamoto@chromium.org,eae
BUG=305885

Review URL: https://codereview.chromium.org/141553003

git-svn-id: svn://svn.chromium.org/blink/trunk@165348 bbb929c8-8fbe-4397-9dbb-9b2b20218538