Tracing V2: Import support for protobuf plugin.

components/tracing/test/example_proto/test_messages.proto

Index: components/tracing/test/example_proto/test_messages.proto
diff --git a/components/tracing/test/example_messages.proto b/components/tracing/test/example_proto/test_messages.proto
similarity index 85%
rename from components/tracing/test/example_messages.proto
rename to components/tracing/test/example_proto/test_messages.proto
index c3210b822c088045cf641fb4e71465a8b141395e..588ec84f9d22ed403ce43c25f79bcd6bce54cee6 100644
--- a/components/tracing/test/example_messages.proto
+++ b/components/tracing/test/example_proto/test_messages.proto
@@ -5,9 +5,17 @@
 syntax = "proto2";
 package foo.bar;
 
+import "library.proto";
+
 // This file contains comprehensive set of supported message structures and
 // data types. Unit tests depends on the plugin-processed version of this file.
 
+// Tests importing message definition from another proto file.
+message TransgalacticParcel {
+  optional TransgalacticMessage message = 1;
+  optional string tracking_code = 2;
+}
+
 enum SmallEnum {
   TO_BE = 1;
   NOT_TO_BE = 0;
@@ -63,4 +71,5 @@ message NestedA {
     optional NestedC value_b = 1;
   }
   repeated NestedB repeated_a = 2;
+  optional NestedB.NestedC super_nested = 3;
 }