Update V8 to r4851 as required by WebKit r61121

Change-Id: Ib01b7c8e38d5b82b254192fc06365aa5b85780c5
diff --git a/src/assembler.cc b/src/assembler.cc
index 871ca86..6a46f61 100644
--- a/src/assembler.cc
+++ b/src/assembler.cc
@@ -449,6 +449,11 @@
       return "external reference";
     case RelocInfo::INTERNAL_REFERENCE:
       return "internal reference";
+    case RelocInfo::DEBUG_BREAK_SLOT:
+#ifndef ENABLE_DEBUGGER_SUPPORT
+      UNREACHABLE();
+#endif
+      return "debug break slot";
     case RelocInfo::NUMBER_OF_MODES:
       UNREACHABLE();
       return "number_of_modes";
@@ -513,6 +518,7 @@
     case STATEMENT_POSITION:
     case EXTERNAL_REFERENCE:
     case INTERNAL_REFERENCE:
+    case DEBUG_BREAK_SLOT:
     case NONE:
       break;
     case NUMBER_OF_MODES: