Special case the "empty string" root so it doesn't constantly jump around

BUG=none
R=hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1075133002

Cr-Commit-Position: refs/heads/master@{#27736}

include/v8.h

@@ -6723,7 +6723,7 @@ class Internals {
   static const int kNullValueRootIndex = 7;
   static const int kTrueValueRootIndex = 8;
   static const int kFalseValueRootIndex = 9;
-  static const int kEmptyStringRootIndex = 156;
+  static const int kEmptyStringRootIndex = 10;
 
   // The external allocation limit should be below 256 MB on all architectures
   // to avoid that resource-constrained embedders run low on memory.

src/heap/heap.cc

@@ -2510,6 +2510,7 @@ const Heap::StringTypeTable Heap::string_type_table[] = {
 
 
 const Heap::ConstantStringTable Heap::constant_string_table[] = {
+    {"", kempty_stringRootIndex},
 #define CONSTANT_STRING_ELEMENT(name, contents) \
   { contents, k##name##RootIndex }              \
   ,

src/heap/heap.h

@@ -37,6 +37,7 @@ namespace internal {
   V(Oddball, null_value, NullValue)                                            \
   V(Oddball, true_value, TrueValue)                                            \
   V(Oddball, false_value, FalseValue)                                          \
+  V(String, empty_string, empty_string)                                        \
   V(Oddball, uninitialized_value, UninitializedValue)                          \
   V(Map, cell_map, CellMap)                                                    \
   V(Map, global_property_cell_map, GlobalPropertyCellMap)                      \
@@ -214,7 +215,6 @@ namespace internal {
   V(constructor_string, "constructor")                         \
   V(dot_result_string, ".result")                              \
   V(eval_string, "eval")                                       \
-  V(empty_string, "")                                          \
   V(function_string, "function")                               \
   V(Function_string, "Function")                               \
   V(length_string, "length")                                   \
@@ -370,6 +370,7 @@ namespace internal {
   V(JSMessageObjectMap)                 \
   V(ForeignMap)                         \
   V(NeanderMap)                         \
+  V(empty_string)                       \
   PRIVATE_SYMBOL_LIST(V)
 
 // Forward declarations.