Fix more -Wsign-compare warnings in heap, mips, base, etc.

test/cctest/test-global-handles.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 449 matching lines @@
 
   v8::Global<v8::Object> g1, g2;
   {
     v8::HandleScope scope(isolate);
     g1.Reset(isolate, v8::Object::New(isolate));
     g1.SetWeak();
     g2.Reset(isolate, v8::Object::New(isolate));
     g2.SetWeak();
   }
 
-  CHECK_EQ(0, isolate->NumberOfPhantomHandleResetsSinceLastCall());
+  CHECK_EQ(0u, isolate->NumberOfPhantomHandleResetsSinceLastCall());
   CcTest::CollectAllAvailableGarbage();
-  CHECK_EQ(2, isolate->NumberOfPhantomHandleResetsSinceLastCall());
-  CHECK_EQ(0, isolate->NumberOfPhantomHandleResetsSinceLastCall());
+  CHECK_EQ(2u, isolate->NumberOfPhantomHandleResetsSinceLastCall());
+  CHECK_EQ(0u, isolate->NumberOfPhantomHandleResetsSinceLastCall());
 }