simplify fExternalCounter bookkeeping

Not sure this fixes anything, or that there's anything wrong here,
but we think this may be easier to reason about.

Change-Id: I08d561760ed6357979205abb9128093adc9cc9a9
Reviewed-on: https://skia-review.googlesource.com/c/177360
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed