Fix some win64 warnings.

src/gpu/GrProgramDesc.h

Index: src/gpu/GrProgramDesc.h
diff --git a/src/gpu/GrProgramDesc.h b/src/gpu/GrProgramDesc.h
index a20b99f74948c91377c7127ef12a17ff683b019b..15734f642db865f01544cb7701a7abc8f6dc976e 100644
--- a/src/gpu/GrProgramDesc.h
+++ b/src/gpu/GrProgramDesc.h
@@ -35,7 +35,7 @@ public:
     uint32_t getChecksum() const { return *this->atOffset<uint32_t, kChecksumOffset>(); }
 
     GrProgramDesc& operator= (const GrProgramDesc& other) {
-        size_t keyLength = other.keyLength();
+        uint32_t keyLength = other.keyLength();
         fKey.reset(keyLength);
         memcpy(fKey.begin(), other.fKey.begin(), keyLength);
         return *this;