change impl to explicitly store length as u32, rather than size_t, so we keep a packed struct on 64…

include/core/SkString.h

Index: include/core/SkString.h
diff --git a/include/core/SkString.h b/include/core/SkString.h
index 743f07889d079c9d41e2644ee4f1c078978671c0..b98e2c8c597763c99ca31c8e4ce7a3275a9740a6 100644
--- a/include/core/SkString.h
+++ b/include/core/SkString.h
@@ -205,7 +205,7 @@ public:
 private:
     struct Rec {
     public:
-        size_t      fLength;
+        uint32_t    fLength; // logically size_t, but we want it to stay 32bits
         int32_t     fRefCnt;
         char        fBeginningOfData;