Turn zone_excess_limit_ into a constant.

The zone_excess_limit_ is not changed at runtime, and there seems to
be no obvious reason to do so in the future.

R=danno@chromium.org
BUG=

Review URL: https://codereview.chromium.org/18034006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15354 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/zone-inl.h

@@ -74,7 +74,7 @@ T* Zone::NewArray(int length) {
 
 
 bool Zone::excess_allocation() {
-  return segment_bytes_allocated_ > zone_excess_limit_;
+  return segment_bytes_allocated_ > kExcessLimit;
 }
 
 

src/zone.cc

@@ -68,8 +68,7 @@ class Segment {
 
 
 Zone::Zone(Isolate* isolate)
-    : zone_excess_limit_(256 * MB),
-      allocation_size_(0),
+    : allocation_size_(0),
       segment_bytes_allocated_(0),
       position_(0),
       limit_(0),

src/zone.h

@@ -91,7 +91,7 @@ class Zone {
   static const int kMaximumSegmentSize = 1 * MB;
 
   // Report zone excess when allocation exceeds this limit.
-  int zone_excess_limit_;
+  static const int kExcessLimit = 256 * MB;
 
   // The number of bytes allocated in this zone so far.
   unsigned allocation_size_;