Add pointer cache field to external string for access in generated code.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 6e672e734aa008662cebe039d63c0c2da355f3c6..343888dd82d7b22552f397be305072a82cd628ec 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -6758,7 +6758,12 @@ class ExternalString: public String {
 
   // Layout description.
   static const int kResourceOffset = POINTER_SIZE_ALIGN(String::kSize);
-  static const int kSize = kResourceOffset + kPointerSize;
+  static const int kResourceDataOffset = kResourceOffset + kPointerSize;
+  static const int kSize = kResourceDataOffset + kPointerSize;
+
+  // Clear the cached pointer to the character array provided by the resource.
+  // This cache is updated the first time the character array is accessed.
+  inline void clear_data_cache();
 
   STATIC_CHECK(kResourceOffset == Internals::kStringResourceOffset);
 
@@ -6779,6 +6784,8 @@ class ExternalAsciiString: public ExternalString {
   inline const Resource* resource();
   inline void set_resource(const Resource* buffer);
 
+  inline const char* GetChars();
+
   // Dispatched behavior.
   uint16_t ExternalAsciiStringGet(int index);
 
@@ -6816,6 +6823,8 @@ class ExternalTwoByteString: public ExternalString {
   inline const Resource* resource();
   inline void set_resource(const Resource* buffer);
 
+  inline const uint16_t* GetChars();
+
   // Dispatched behavior.
   uint16_t ExternalTwoByteStringGet(int index);