Fix -Wshorten-64-to-32 in test-assembler-arm64.cc

The arm64 MacroAssembler expects buffer_size to be an unsigned, not a
size_t.

BUG=chromium:710913

Review-Url: https://codereview.chromium.org/2818513002
Cr-Commit-Position: refs/heads/master@{#44623}

test/cctest/test-assembler-arm64.cc

@@ -173,15 +173,15 @@ static void InitializeVM() {
 
 #else  // ifdef USE_SIMULATOR.
 // Run the test on real hardware or models.
-#define SETUP_SIZE(buf_size)                                   \
-  Isolate* isolate = CcTest::i_isolate();                      \
-  HandleScope scope(isolate);                                  \
-  CHECK(isolate != NULL);                                      \
-  size_t actual_size;                                          \
-  byte* buf = static_cast<byte*>(                              \
-      v8::base::OS::Allocate(buf_size, &actual_size, true));   \
-  MacroAssembler masm(isolate, buf, actual_size,               \
-                      v8::internal::CodeObjectRequired::kYes); \
+#define SETUP_SIZE(buf_size)                                            \
+  Isolate* isolate = CcTest::i_isolate();                               \
+  HandleScope scope(isolate);                                           \
+  CHECK(isolate != NULL);                                               \
+  size_t actual_size;                                                   \
+  byte* buf = static_cast<byte*>(                                       \
+      v8::base::OS::Allocate(buf_size, &actual_size, true));            \
+  MacroAssembler masm(isolate, buf, static_cast<unsigned>(actual_size), \
+                      v8::internal::CodeObjectRequired::kYes);          \
   RegisterDump core;
 
 #define RESET()                                                                \