Hold dex caches live in class table

Prevents temporary dex caches being unloaded for the same dex file.
Usually this is OK, but if someone resolved a string in that dex
cache, it could leave stale pointers in BSS. Also it can use extra
memory in linear alloc if we allocate dex cache arrays multiple
times.

Bug: 29083330
Change-Id: Ia44668f013ceef1f5eb80f653a48d0f8004548c9
diff --git a/runtime/class_table.cc b/runtime/class_table.cc
index 8267c68..f5ebcc5 100644
--- a/runtime/class_table.cc
+++ b/runtime/class_table.cc
@@ -146,15 +146,15 @@
   return ComputeModifiedUtf8Hash(descriptor);
 }
 
-bool ClassTable::InsertDexFile(mirror::Object* dex_file) {
+bool ClassTable::InsertStrongRoot(mirror::Object* obj) {
   WriterMutexLock mu(Thread::Current(), lock_);
-  DCHECK(dex_file != nullptr);
-  for (GcRoot<mirror::Object>& root : dex_files_) {
-    if (root.Read() == dex_file) {
+  DCHECK(obj != nullptr);
+  for (GcRoot<mirror::Object>& root : strong_roots_) {
+    if (root.Read() == obj) {
       return false;
     }
   }
-  dex_files_.push_back(GcRoot<mirror::Object>(dex_file));
+  strong_roots_.push_back(GcRoot<mirror::Object>(obj));
   return true;
 }