Re-add dot_result_string to heap.h after 5f7b24f7

Failed to notice it was still being used in a DCHECK, so removing
it broke the debug build.

TBR=marja@chromium.org

Review URL: https://codereview.chromium.org/720793004

Cr-Commit-Position: refs/heads/master@{#25309}

include/v8.h

@@ -6118,7 +6118,7 @@ class Internals {
   static const int kNullValueRootIndex = 7;
   static const int kTrueValueRootIndex = 8;
   static const int kFalseValueRootIndex = 9;
-  static const int kEmptyStringRootIndex = 153;
+  static const int kEmptyStringRootIndex = 154;
 
   // 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.h

@@ -209,6 +209,7 @@ namespace internal {
   V(Boolean_string, "Boolean")                             \
   V(callee_string, "callee")                               \
   V(constructor_string, "constructor")                     \
+  V(dot_result_string, ".result")                          \
   V(eval_string, "eval")                                   \
   V(empty_string, "")                                      \
   V(function_string, "function")                           \