[heap] Use size_t for heap and space counters.

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index d202aadf7a141b167cf6a1bc608e2bc841b948f8..5402a8ce87a3987981e1c8a33340238b95b517c3 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -402,8 +402,9 @@ void Assembler::GrowBuffer() {
 
   // Some internal data structures overflow for very large buffers,
   // they must ensure that kMaximalBufferSize is not too large.
-  if ((desc.buffer_size > kMaximalBufferSize) ||
-      (desc.buffer_size > isolate()->heap()->MaxOldGenerationSize())) {
+  if (desc.buffer_size > kMaximalBufferSize ||
+      static_cast<size_t>(desc.buffer_size) >
+          isolate()->heap()->MaxOldGenerationSize()) {
     V8::FatalProcessOutOfMemory("Assembler::GrowBuffer");
   }