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

@@ -180,7 +180,7 @@ static void InitializeVM() {
   size_t actual_size;                                                   \
   byte* buf = static_cast<byte*>(                                       \
       v8::base::OS::Allocate(buf_size, &actual_size, true));            \
-  MacroAssembler masm(isolate, buf, actual_size,               \
+  MacroAssembler masm(isolate, buf, static_cast<unsigned>(actual_size), \
                       v8::internal::CodeObjectRequired::kYes);          \
   RegisterDump core;