Update SkTypefaceCache.h

Only force purge all when clearing singleton interface

Fix for CYAN-4047

Always forcing the typeface cache to be purged can lead to system
lock ups.  It should only be forced because of a font change, in
which case we want the entire cache purged and then reloaded with
the new fonts.  Calling this any other time should have force set
to false, i.e. when Canvas calls PurgeAll().

Change-Id: Ibdeeeaa1ee67270da365b69c63c9c4914d60374e
Merge pull request #2 from spillow64/patch-2

Update SkTypefaceCache.h