ART: Fix GetThreadInfo

Fix the retrieval of the context classloader. Make sure to have
the field from the Thread class, don't look into a subclass.

Add caching for the field. Also fix the usage restriction of the
function, which is only valid during the Live phase.

Bug: 36654185
Test: ./test.py --host -r -t 924
Change-Id: I5cce41b31c32e59c80bb7c3afa03e8b0975ec54e
diff --git a/runtime/openjdkjvmti/ti_thread.h b/runtime/openjdkjvmti/ti_thread.h
index f6f93ee..c7f75d8 100644
--- a/runtime/openjdkjvmti/ti_thread.h
+++ b/runtime/openjdkjvmti/ti_thread.h
@@ -35,6 +35,10 @@
 #include "jni.h"
 #include "jvmti.h"
 
+namespace art {
+class ArtField;
+}
+
 namespace openjdkjvmti {
 
 class EventHandler;
@@ -44,6 +48,9 @@
   static void Register(EventHandler* event_handler);
   static void Unregister();
 
+  // To be called when it is safe to cache data.
+  static void CacheData();
+
   static jvmtiError GetAllThreads(jvmtiEnv* env, jint* threads_count_ptr, jthread** threads_ptr);
 
   static jvmtiError GetCurrentThread(jvmtiEnv* env, jthread* thread_ptr);
@@ -60,6 +67,9 @@
                                    jvmtiStartFunction proc,
                                    const void* arg,
                                    jint priority);
+
+ private:
+  static art::ArtField* context_class_loader_;
 };
 
 }  // namespace openjdkjvmti