Update V8 to r5388 as required by WebKit r66666

Change-Id: Ib3c42e9b7226d22c65c7077c543fe31afe62a318
diff --git a/src/conversions.h b/src/conversions.h
index c4ceea6..9e32a0c 100644
--- a/src/conversions.h
+++ b/src/conversions.h
@@ -96,8 +96,12 @@
 
 
 // Converts a string into a double value according to ECMA-262 9.3.1
-double StringToDouble(const char* str, int flags, double empty_string_val = 0);
 double StringToDouble(String* str, int flags, double empty_string_val = 0);
+double StringToDouble(Vector<const char> str,
+                      int flags,
+                      double empty_string_val = 0);
+// This version expects a zero-terminated character array.
+double StringToDouble(const char* str, int flags, double empty_string_val = 0);
 
 // Converts a string into an integer.
 double StringToInt(String* str, int radix);