Fix String::DoReplace() using obsolete `this`.

Change it to a static function taking a Handle<>.

Test: testrunner.py --host --interp-ac --gcstress -t 021-string2
Bug: 36335996
Change-Id: I5ab3e7adc59d6a9095290e57d5ce5d46b79f089b
diff --git a/runtime/mirror/string.h b/runtime/mirror/string.h
index dbb5a4c..b59bbfb 100644
--- a/runtime/mirror/string.h
+++ b/runtime/mirror/string.h
@@ -96,7 +96,7 @@
 
   // Create a new string where all occurences of `old_c` are replaced with `new_c`.
   // String.doReplace(char, char) is called from String.replace(char, char) when there is a match.
-  ObjPtr<String> DoReplace(Thread* self, uint16_t old_c, uint16_t new_c)
+  static ObjPtr<String> DoReplace(Thread* self, Handle<String> src, uint16_t old_c, uint16_t new_c)
       REQUIRES_SHARED(Locks::mutator_lock_);
 
   ObjPtr<String> Intern() REQUIRES_SHARED(Locks::mutator_lock_);