A64: Fix compilation with clang

The size_ member is only used in debug mode, so we should only define
it in debug mode.

BUG=none
R=rodolph.perfetta@arm.com, ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/171583004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19495 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/a64/macro-assembler-a64.h

@@ -2166,7 +2166,12 @@ class MacroAssembler : public Assembler {
 class InstructionAccurateScope BASE_EMBEDDED {
  public:
   InstructionAccurateScope(MacroAssembler* masm, size_t count = 0)
-      : masm_(masm), size_(count * kInstructionSize) {
+      : masm_(masm)
+#ifdef DEBUG
+        ,
+        size_(count * kInstructionSize)
+#endif
+  {
     // Before blocking the const pool, see if it needs to be emitted.
     masm_->CheckConstPool(false, true);
 
@@ -2192,8 +2197,8 @@ class InstructionAccurateScope BASE_EMBEDDED {
 
  private:
   MacroAssembler* masm_;
-  size_t size_;
 #ifdef DEBUG
+  size_t size_;
   Label start_;
   bool previous_allow_macro_instructions_;
 #endif