Add guard pages in front of platform allocations

src/spaces.h

Index: src/spaces.h
===================================================================
--- src/spaces.h	(revision 8619)
+++ src/spaces.h	(working copy)
@@ -662,18 +662,26 @@
   // If a chunk has at least 16 pages, the maximum heap size is about
   // 8K * 8K * 16 = 1G bytes.
 #ifdef V8_TARGET_ARCH_X64
-  static const int kPagesPerChunk = 32;
+#ifdef ENABLE_HEAP_PROTECTION
+  static const int kPagesPerChunk = 31;
+#else  // ENABLE_HEAP_PROTECTION
+    static const int kPagesPerChunk = 32;
+#endif  // ENABLE_HEAP_PROTECTION
   // On 64 bit the chunk table consists of 4 levels of 4096-entry tables.
   static const int kPagesPerChunkLog2 = 5;
   static const int kChunkTableLevels = 4;
   static const int kChunkTableBitsPerLevel = 12;
 #else
-  static const int kPagesPerChunk = 16;
+#ifdef ENABLE_HEAP_PROTECTION
+  static const int kPagesPerChunk = 15;
+#else  // ENABLE_HEAP_PROTECTION
+    static const int kPagesPerChunk = 16;
+#endif  // ENABLE_HEAP_PROTECTION
   // On 32 bit the chunk table consists of 2 levels of 256-entry tables.
   static const int kPagesPerChunkLog2 = 4;
   static const int kChunkTableLevels = 2;
   static const int kChunkTableBitsPerLevel = 8;
-#endif
+#endif  // V8_TARGET_ARCH_X64
 
  private:
   MemoryAllocator();