[wasm] Fix -Wsign-compare warnings.

test/unittests/wasm/module-decoder-unittest.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "test/unittests/test-utils.h"
 
 #include "src/handles.h"
 #include "src/objects-inl.h"
 #include "src/wasm/module-decoder.h"
 #include "src/wasm/wasm-macro-gen.h"
@@ skipping 1186 matching lines @@
       // signatures
       SIGNATURES_SECTION_VOID_VOID,
       ONE_EMPTY_FUNCTION,
       SECTION(Export, 1 + 6),
       1,        // exports
       NO_NAME,  // --
       kExternalFunction,
       FUNC_INDEX(0),  // --
   };
 
-  for (int length = 33; length < sizeof(data); length++) {
+  for (size_t length = 33; length < sizeof(data); length++) {
     ModuleResult result = DecodeModule(data, data + length);
     EXPECT_FALSE(result.ok());
     if (result.val) delete result.val;
   }
 }
 
 TEST_F(WasmModuleVerifyTest, FunctionSignatures_empty) {
   static const byte data[] = {
       SECTION(Type, 1), 0,     // --
       SECTION(Function, 1), 0  // --
@@ skipping 172 matching lines @@
       SECTION(Unknown, 4), 1, 'X', 17,  18,                      // --
       SECTION(Unknown, 9), 3, 'f', 'o', 'o', 5,   6,   7, 8, 9,  // --
       SECTION(Unknown, 8), 5, 'o', 't', 'h', 'e', 'r', 7, 8,     // --
   };
   EXPECT_VERIFIES(data);
 }
 
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8