Fix some win64 warnings.

src/core/SkVarAlloc.h

 #ifndef SkVarAlloc_DEFINED
 #define SkVarAlloc_DEFINED
 
 #include "SkTypes.h"
 
 class SkVarAlloc : SkNoncopyable {
 public:
     SkVarAlloc();
     ~SkVarAlloc();
 
     // Returns contiguous bytes aligned at least for pointers.  You may pass SK_MALLOC_THROW, etc.
     char* alloc(size_t bytes, unsigned sk_malloc_flags) {
         bytes = SkAlignPtr(bytes);
 
         if (bytes > fRemaining) {
             this->makeSpace(bytes, sk_malloc_flags);
         }
         SkASSERT(bytes <= fRemaining);
 
         char* ptr = fByte;
         fByte += bytes;
-        fRemaining -= bytes;
+        fRemaining = SkToU32(fRemaining - bytes);
         return ptr;
     }
 
     // Returns our best estimate of the number of bytes we've allocated.
     // (We intentionally do not track this precisely to save space.)
     size_t approxBytesAllocated() const;
 
 private:
     void makeSpace(size_t bytes, unsigned flags);
 
     char* fByte;
     unsigned fRemaining;
     unsigned fLgSize;  // This is always in the range [4, 16], so it really only needs 4 bits.
 
     struct Block;
     Block* fBlock;
 };
 SK_COMPILE_ASSERT(sizeof(SkVarAlloc) <= 24, SkVarAllocSize);
 
 #endif//SkVarAlloc_DEFINED