Clean up CSSFontFaceSource

Several minor refactorings:
- remove unused parameter of getFontData()
- kill willUseFontData()
- rename beginLoadingFontSoon() to beginLoadIfNeeded()
- make pruneTable() private

BUG=
TEST=none, no behavior change

Review URL: https://codereview.chromium.org/94393002

git-svn-id: svn://svn.chromium.org/blink/trunk@162943 bbb929c8-8fbe-4397-9dbb-9b2b20218538