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/trace.cc b/runtime/trace.cc
index b30de79..4c3fa20 100644
--- a/runtime/trace.cc
+++ b/runtime/trace.cc
@@ -832,6 +832,11 @@
              << " " << dex_pc;
 }
 
+void Trace::WatchedFramePop(Thread* self ATTRIBUTE_UNUSED,
+                            const ShadowFrame& frame ATTRIBUTE_UNUSED) {
+  LOG(ERROR) << "Unexpected WatchedFramePop event in tracing";
+}
+
 void Trace::ReadClocks(Thread* thread, uint32_t* thread_clock_diff, uint32_t* wall_clock_diff) {
   if (UseThreadCpuClock()) {
     uint64_t clock_base = thread->GetTraceClockBase();