Remove hack allowing fRefCnt of 0.

In some legacy situations users of SkRefCnt subclasses were keeping
the objects alive with a reference count of 0. Now that these users are
cleaned up, remove the hack which allowed such code to keep functioning.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3264

Change-Id: I22f63d87b6d995cad6326998284930ad9eaa2983
Reviewed-on: https://skia-review.googlesource.com/3264
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

include/core/SkRefCnt.h

@@ -71,15 +71,7 @@ public:
     /** Increment the reference count. Must be balanced by a call to unref().
     */
     void ref() const {
-#ifdef SK_BUILD_FOR_ANDROID_FRAMEWORK
-        // Android employs some special subclasses that enable the fRefCnt to
-        // go to zero, but not below, prior to reusing the object.  This breaks
-        // the use of unique() on such objects and as such should be removed
-        // once the Android code is fixed.
-        SkASSERT(getRefCnt() >= 0);
-#else
         SkASSERT(getRefCnt() > 0);
-#endif
         // No barrier required.
         (void)fRefCnt.fetch_add(+1, std::memory_order_relaxed);
     }