[Font Load Events] Refactoring: remove FontFaceSet.m_shouldFireDoneEvent

Replace m_shouldFireDoneEvent field with hasLoadedFonts() method that
checks pending loaded/failed fonts array.
There is no need of a boolean field to track the state.

BUG=none
TEST=No behavior change, covered by existing test

Review URL: https://codereview.chromium.org/36963002

git-svn-id: svn://svn.chromium.org/blink/trunk@160794 bbb929c8-8fbe-4397-9dbb-9b2b20218538