Let store buffer start out small for a 1Mbyte saving in boot

test/cctest/test-mark-compact.cc

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 510 matching lines @@
 }
 
 
 TEST(BootUpMemoryUse) {
   intptr_t initial_memory = MemoryInUse();
   // Only Linux has the proc filesystem and only if it is mapped.  If it's not
   // there we just skip the test.
   if (initial_memory >= 0) {
     InitializeVM();
     intptr_t booted_memory = MemoryInUse();
-    CHECK_LE(booted_memory - initial_memory, 20 * 1024 * 1024);
+    CHECK_LE(booted_memory - initial_memory, 18 * 1024 * 1024);
   }
 }
 
 #endif  // __linux__