remove Sk64 from public API, and start to remove usage internally

src/core/SkMallocPixelRef.cpp

Index: src/core/SkMallocPixelRef.cpp
diff --git a/src/core/SkMallocPixelRef.cpp b/src/core/SkMallocPixelRef.cpp
index b65197fac852ae7e5dceb3bf4ed4764dc92cd087..9e2ea9c9803c6475b02263325a9bc2fbbc958726 100644
--- a/src/core/SkMallocPixelRef.cpp
+++ b/src/core/SkMallocPixelRef.cpp
@@ -64,13 +64,12 @@ SkMallocPixelRef* SkMallocPixelRef::NewAllocate(const SkImageInfo& info,
         rowBytes = minRB;
     }
 
-    Sk64 bigSize;
-    bigSize.setMul(info.fHeight, rowBytes);
-    if (!bigSize.is32()) {
+    int64_t bigSize = (int64_t)info.fHeight * rowBytes;
+    if (!sk_64_isS32(bigSize)) {
         return NULL;
     }
 
-    size_t size = bigSize.get32();
+    size_t size = sk_64_asS32(bigSize);
     void* addr = sk_malloc_flags(size, 0);
     if (NULL == addr) {
         return NULL;