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/debugger.cc b/runtime/debugger.cc
index af56810..b4931cf 100644
--- a/runtime/debugger.cc
+++ b/runtime/debugger.cc
@@ -268,6 +268,11 @@
     LOG(ERROR) << "Unexpected exception handled event in debugger";
   }
 
+  // TODO Might be worth it to implement this.
+  void WatchedFramePop(Thread* thread ATTRIBUTE_UNUSED,
+                       const ShadowFrame& frame ATTRIBUTE_UNUSED) OVERRIDE {
+    LOG(ERROR) << "Unexpected WatchedFramePop event in debugger";
+  }
 
  private:
   static bool IsReturn(ArtMethod* method, uint32_t dex_pc)