Add scoped trace to VisitDexCacheArrays

Diagnose how expensive dex cache relocation is.

Bug: 116052292
Test: manual

Change-Id: I032b72de0db61932bc6e6c4da2a910ce9b2d1939
diff --git a/runtime/gc/space/image_space.cc b/runtime/gc/space/image_space.cc
index b929560..c4f1c2e 100644
--- a/runtime/gc/space/image_space.cc
+++ b/runtime/gc/space/image_space.cc
@@ -548,6 +548,7 @@
 
   void VisitDexCacheArrays(ObjPtr<mirror::DexCache> dex_cache)
       REQUIRES_SHARED(Locks::mutator_lock_) {
+    ScopedTrace st("VisitDexCacheArrays");
     FixupDexCacheArray<mirror::StringDexCacheType>(dex_cache,
                                                    mirror::DexCache::StringsOffset(),
                                                    dex_cache->NumStrings<kVerifyNone>());