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

src/images/SkImageDecoder_libwebp.cpp

Index: src/images/SkImageDecoder_libwebp.cpp
diff --git a/src/images/SkImageDecoder_libwebp.cpp b/src/images/SkImageDecoder_libwebp.cpp
index ab58aef135baa4ae6b4c0bae267e3b4fb7d42199..84c438affc6cda16852f9d13ebdb4f1f876a6a73 100644
--- a/src/images/SkImageDecoder_libwebp.cpp
+++ b/src/images/SkImageDecoder_libwebp.cpp
@@ -80,13 +80,12 @@ static bool webp_parse_header(SkStream* stream, int* width, int* height, int* al
 
     // sanity check for image size that's about to be decoded.
     {
-        Sk64 size;
-        size.setMul(*width, *height);
-        if (size.isNeg() || !size.is32()) {
+        int64_t size = sk_64_mul(*width, *height);
+        if (!sk_64_isS32(size)) {
             return false;
         }
         // now check that if we are 4-bytes per pixel, we also don't overflow
-        if (size.get32() > (0x7FFFFFFF >> 2)) {
+        if (sk_64_asS32(size) > (0x7FFFFFFF >> 2)) {
             return false;
         }
     }