Add explicit casts from size_t to uint32_t

x64 MSVC is unhappy with the implict conversions.

R=viettrungluu@chromium.org, azani@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1155973004
diff --git a/mojo/public/cpp/bindings/tests/union_unittest.cc b/mojo/public/cpp/bindings/tests/union_unittest.cc
index 65a23fa..e6e38de 100644
--- a/mojo/public/cpp/bindings/tests/union_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/union_unittest.cc
@@ -499,7 +499,8 @@
   Serialize_(small_struct.Pass(), &buf, &data);
 
   void* raw_buf = buf.Leak();
-  mojo::internal::BoundsChecker bounds_checker(data, size, 0);
+  mojo::internal::BoundsChecker bounds_checker(data,
+                                               static_cast<uint32_t>(size), 0);
   EXPECT_TRUE(internal::SmallStruct_Data::Validate(raw_buf, &bounds_checker));
   free(raw_buf);
 }
@@ -519,7 +520,8 @@
   data->pod_union.tag = static_cast<internal::PodUnion_Data::PodUnion_Tag>(100);
 
   void* raw_buf = buf.Leak();
-  mojo::internal::BoundsChecker bounds_checker(data, size, 0);
+  mojo::internal::BoundsChecker bounds_checker(data,
+                                               static_cast<uint32_t>(size), 0);
   EXPECT_FALSE(internal::SmallStruct_Data::Validate(raw_buf, &bounds_checker));
   free(raw_buf);
 }
@@ -537,7 +539,8 @@
       internal::SmallStructNonNullableUnion_Data::New(&buf);
 
   void* raw_buf = buf.Leak();
-  mojo::internal::BoundsChecker bounds_checker(data, size, 0);
+  mojo::internal::BoundsChecker bounds_checker(data,
+                                               static_cast<uint32_t>(size), 0);
   EXPECT_FALSE(internal::SmallStructNonNullableUnion_Data::Validate(
       raw_buf, &bounds_checker));
   free(raw_buf);
@@ -555,7 +558,8 @@
   Serialize_(small_struct.Pass(), &buf, &data);
 
   void* raw_buf = buf.Leak();
-  mojo::internal::BoundsChecker bounds_checker(data, size, 0);
+  mojo::internal::BoundsChecker bounds_checker(data,
+                                               static_cast<uint32_t>(size), 0);
   EXPECT_TRUE(internal::SmallStruct_Data::Validate(raw_buf, &bounds_checker));
   free(raw_buf);
 }