Implement sk_atomic_conditional_inc with sk_atomic_cas.

src/ports/SkAtomics_sync.h

Index: src/ports/SkAtomics_sync.h
diff --git a/src/ports/SkAtomics_sync.h b/src/ports/SkAtomics_sync.h
index 9a4fe7a6e65f347ab8c91a8306d87eefb08b7c14..45ba63f305c5193dee30eca074123840b0811936 100644
--- a/src/ports/SkAtomics_sync.h
+++ b/src/ports/SkAtomics_sync.h
@@ -26,24 +26,6 @@ static inline __attribute__((always_inline)) int32_t sk_atomic_dec(int32_t* addr
 
 static inline __attribute__((always_inline)) void sk_membar_acquire__after_atomic_dec() { }
 
-static inline __attribute__((always_inline)) int32_t sk_atomic_conditional_inc(int32_t* addr) {
-    int32_t value = *addr;
-
-    while (true) {
-        if (value == 0) {
-            return 0;
-        }
-
-        int32_t before = __sync_val_compare_and_swap(addr, value, value + 1);
-
-        if (before == value) {
-            return value;
-        } else {
-            value = before;
-        }
-    }
-}
-
 static inline __attribute__((always_inline)) bool sk_atomic_cas(int32_t* addr,
                                                                 int32_t before,
                                                                 int32_t after) {