arc: enable Android tracing in verified-boot mode

components/arc/common/tracing.mojom

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Next MinVersion: 1

[Luis Héctor Chávez, 2017/04/13 21:37:52]

Next MinVersion: 2

[Earl Ou, 2017/04/15 20:05:51]

Done.

 
 module arc.mojom;
 
 interface TracingInstance {
   // Queries available tracing categories in the container.
   QueryAvailableCategories@0() => (array<string> categories);
 
-  // Starts tracing in the container with the given categories.
-  StartTracing@1(array<string> categories) => (bool success);
+  // Starts tracing in the container with the given categories. A handle is
+  // passed to the client for sending trace events back to the host. The client
+  // should send trace event as a JSON object in each write.
+  StartTracing@1(array<string> categories, handle socket) => (bool success);

[Luis Héctor Chávez, 2017/04/13 21:37:52]

Oh! I almost forgot!

[MinVersion=1] handle socket

[Earl Ou, 2017/04/14 22:37:38]

This would be a mojom compiling error:
Exception: Non-nullable fields are only allowed in version 0 of a struct.
TracingInstance_StartTracing_Params.socket is defined with [MinVersion=1]

Do you think I should do something like:

DeprecatedStartTracing@1(array<string> categories) => (bool success);
[MinVersion=1] StartTracing@3(array<string> categories, handle socket) => (bool
success);

[Luis Héctor Chávez, 2017/04/15 05:34:19]

no, that's what the versions are for :)

i messed up, it should have been

 [MinVersion=1] handle? socket

[Earl Ou, 2017/04/15 20:05:51]

Done. That works. :)

 
   // Stops tracing in the container.
   StopTracing@2() => (bool success);
 };