Tracing v2: wire up stubs generated by the protozero protoc plugin

components/tracing/core/trace_buffer_writer.h

Index: components/tracing/core/trace_buffer_writer.h
diff --git a/components/tracing/core/trace_buffer_writer.h b/components/tracing/core/trace_buffer_writer.h
index 17ae27145e461e1f98f6c97a956a576dcd42f630..2a6b5e0bf9327d6fbc6a7f140a23740dcfbb9892 100644
--- a/components/tracing/core/trace_buffer_writer.h
+++ b/components/tracing/core/trace_buffer_writer.h
@@ -12,17 +12,19 @@
 #include "components/tracing/core/trace_ring_buffer.h"
 #include "components/tracing/tracing_export.h"
 
+// Fwd declaration from the generated components/tracing/proto/event.pbzero.h.
+namespace pbzero {
 namespace tracing {
-
-// TODO(primiano): in next CL the Event class will just come from the C++
-// header generated by the ProtoZero plugin.
 namespace proto {
-class Event : public v2::ProtoZeroMessage {};
-}
+class Event;
+}  // namespace proto
+}  // namespace tracing
+}  // namespace pbzero
 
+namespace tracing {
 namespace v2 {
 
-using TraceEventHandle = ProtoZeroMessageHandle<::tracing::proto::Event>;
+using TraceEventHandle = ProtoZeroMessageHandle<pbzero::tracing::proto::Event>;
 
 // This class is the entry-point to add events to the TraceRingBuffer.
 // It acts as a glue layer between the protobuf classes (ProtoZeroMessage) and