Turn the utility process image decoder into a Mojo service.

chrome/utility/chrome_content_utility_client.h

Index: chrome/utility/chrome_content_utility_client.h
diff --git a/chrome/utility/chrome_content_utility_client.h b/chrome/utility/chrome_content_utility_client.h
index f2eee64331e8188c2265647d196bc487379fc8eb..23c328b59e2d207b77725b9ae60ac7fb3126e114 100644
--- a/chrome/utility/chrome_content_utility_client.h
+++ b/chrome/utility/chrome_content_utility_client.h
@@ -42,6 +42,14 @@ class ChromeContentUtilityClient : public content::ContentUtilityClient {
     max_ipc_message_size_ = max_message_size;
   }
 
+  // Reference counting for Mojo services. A reference count is incremented when
+  // Mojo services are connected to, and decremented when the connection is
+  // destroyed. When the count reaches 0, this process is freed. The assumption
+  // is that multiple Mojo services can be connected to in the same utility
+  // process. This is a Mojo alternative to "batch mode" for Chrome IPC.
+  void MojoRef();

[sky, 2015/04/01 15:00:45]

Who is responsible for invoking these?

[Anand Mistry (off Chromium), 2015/04/02 04:29:45]

Added a blurb to describe this. At this point, I'm sure I want to refcount the
Mojo connections for establishing the lifetime of the utility process/thread.
However, the exact mechanism for doing so is something I'm still grappling with.
I added some templating to chrome_content_utility_client.cc which does it
implicitly based on a service implementation's lifetime, assuming a
StrongBinding<>. But I've also thought about doing it by passing a callback to
the service. I'm not sure about this yet.

[sky, 2015/04/02 17:20:10]

Having to explicitly invoke functions like this is error prone. Make it
consumers have to use a RAII class whose constructor does the Ref and destructor
the Unref.

[Anand Mistry (off Chromium), 2015/04/08 07:14:15]

On further thought, there are races with this kind of service tracking and
shutdown which can cause spurious failures at the consumers. For now, I think
it's best to leave this out and require the browser side to shut down the
utility process when connecting to Mojo-based services.

Also, if the utility process is untrusted, can we really trust it to shut itself
down?

+  void MojoUnref();
+
  private:
   // IPC message handlers.
   void OnUnpackWebResource(const std::string& resource_data);
@@ -89,6 +97,8 @@ class ChromeContentUtilityClient : public content::ContentUtilityClient {
   // Maximum IPC msg size (default to kMaximumMessageSize; override for testing)
   static int64_t max_ipc_message_size_;
 
+  int32_t mojo_service_ref_count_;
+
   DISALLOW_COPY_AND_ASSIGN(ChromeContentUtilityClient);
 };