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

src/core/SkMipMap.cpp

Index: src/core/SkMipMap.cpp
diff --git a/src/core/SkMipMap.cpp b/src/core/SkMipMap.cpp
index ff62f4d59d53b154340d69a611621fec49edc1c7..4888b574e4a687c63e400374b99d4c973686535b 100644
--- a/src/core/SkMipMap.cpp
+++ b/src/core/SkMipMap.cpp
@@ -109,15 +109,21 @@
     *dst->getAddr16(x >> 1, y >> 1) = (uint16_t)collaps4444(c >> 2);
 }
 
+static bool isPos32Bits(const Sk64& value) {
+    return !value.isNeg() && value.is32();
+}
+
 SkMipMap::Level* SkMipMap::AllocLevels(int levelCount, size_t pixelSize) {
     if (levelCount < 0) {
         return NULL;
     }
-    int64_t size = sk_64_mul(levelCount + 1, sizeof(Level)) + pixelSize;
-    if (!sk_64_isS32(size)) {
-        return NULL;
-    }
-    return (Level*)sk_malloc_throw(sk_64_asS32(size));
+    Sk64 size;
+    size.setMul(levelCount + 1, sizeof(Level));
+    size.add(SkToS32(pixelSize));
+    if (!isPos32Bits(size)) {
+        return NULL;
+    }
+    return (Level*)sk_malloc_throw(size.get32());
 }
 
 SkMipMap* SkMipMap::Build(const SkBitmap& src) {