Trim FontSelector interface a bit.

A two-parter:
1) Split off FontCacheClient interface and make FontCache forget about
FontSelector altogether.
2) De-interface font invalidation callback registration/unregisration,
since they are only used in Source/core. This costs us a couple of
static_casts, but overall, it still looks cleaner.

R=ksakamoto@chromium.org,eae
BUG=315783

Review URL: https://codereview.chromium.org/140913011

git-svn-id: svn://svn.chromium.org/blink/trunk@165351 bbb929c8-8fbe-4397-9dbb-9b2b20218538