Simplify FontFaceSet's pending operations handling

- Since promises' callbacks are invoked asynchronously, no need to make
  LoadFontPromiseResolver::resolve() async.
- We have at most one pending "loading" event. Replace Vector<<Event>
  with a bool.
- Rename FontsReadyPromiseResolver::call() to resolve().

BUG=
TEST=none, just refactoring

Review URL: https://codereview.chromium.org/93893002

git-svn-id: svn://svn.chromium.org/blink/trunk@162940 bbb929c8-8fbe-4397-9dbb-9b2b20218538