Revert "Revert "Make WatchedFramePop instrumentation event pure-virtual""

Forgot to revert this.

This reverts commit b2c8fc62f4e7889300da2d26294537194e37b926.

Reason for revert: Fixed issue with tests

Change-Id: I1fd5a2759c67a906d55137e2b6548726af2c8e4f
Test: treehugger
diff --git a/runtime/instrumentation.h b/runtime/instrumentation.h
index fec027e..9bb49ea 100644
--- a/runtime/instrumentation.h
+++ b/runtime/instrumentation.h
@@ -154,9 +154,7 @@
   // shadow-frames by deoptimizing stacks.
   virtual void WatchedFramePop(Thread* thread ATTRIBUTE_UNUSED,
                                const ShadowFrame& frame ATTRIBUTE_UNUSED)
-      REQUIRES_SHARED(Locks::mutator_lock_) {
-    return;
-  }
+      REQUIRES_SHARED(Locks::mutator_lock_) = 0;
 };
 
 // Instrumentation is a catch-all for when extra information is required from the runtime. The