Prevent ArtMethod clearing race

There was race condition where the GC would be visiting the methods
of a class while the class linker overwrote the contents with 0xFE
when copying to a new array.

Since the GC is holding the class table lock at this time, we can
use this lock in the class linker to prevent the race.

Bug: 28699001

(cherry picked from commit 10c5f56423feaf3eadb3d4c09c61d2b998404162)

Change-Id: I5ddca93106cb8e48962cf44e5ce434c45e05cd0c
diff --git a/runtime/class_linker.h b/runtime/class_linker.h
index 1845f44..9b5ca0e 100644
--- a/runtime/class_linker.h
+++ b/runtime/class_linker.h
@@ -1024,7 +1024,7 @@
 
   // Returns null if not found.
   ClassTable* ClassTableForClassLoader(mirror::ClassLoader* class_loader)
-      SHARED_REQUIRES(Locks::mutator_lock_, Locks::classlinker_classes_lock_);
+      SHARED_REQUIRES(Locks::mutator_lock_);
 
   // Insert a new class table if not found.
   ClassTable* InsertClassTableForClassLoader(mirror::ClassLoader* class_loader)