Use an explicit PID for duplicating Pepper handles rather than the Channel's.

ppapi/proxy/proxy_channel.h

Index: ppapi/proxy/proxy_channel.h
diff --git a/ppapi/proxy/proxy_channel.h b/ppapi/proxy/proxy_channel.h
index bb08f79c7f642c5ef9ac5cea80d9f6d8a3c9a653..d35f98a8628fcd9f19d88591e670b376ee7cb706 100644
--- a/ppapi/proxy/proxy_channel.h
+++ b/ppapi/proxy/proxy_channel.h
@@ -47,7 +47,7 @@ class PPAPI_PROXY_EXPORT ProxyChannel
     // guarantees as ProxyChannel::ShareHandleWithRemote below.
     virtual IPC::PlatformFileForTransit ShareHandleWithRemote(
         base::PlatformFile handle,
-        const IPC::SyncChannel& channel,
+        base::ProcessId remote_pid,
         bool should_close_source) = 0;
   };
 
@@ -55,7 +55,8 @@ class PPAPI_PROXY_EXPORT ProxyChannel
 
   // Alternative to InitWithChannel() for unit tests that want to send all
   // messages sent via this channel to the given test sink. The test sink
-  // must outlive this class.
+  // must outlive this class. In this case, the peer PID will be the current
+  // process ID.
   void InitWithTestSink(IPC::TestSink* test_sink);
 
   // Shares a file handle (HANDLE / file descriptor) with the remote side. It
@@ -90,6 +91,7 @@ class PPAPI_PROXY_EXPORT ProxyChannel
   // The delegate pointer must outlive this class, ownership is not
   // transferred.
   virtual bool InitWithChannel(Delegate* delegate,
+                               base::ProcessId peer_pid,
                                const IPC::ChannelHandle& channel_handle,
                                bool is_client);
 
@@ -101,6 +103,11 @@ class PPAPI_PROXY_EXPORT ProxyChannel
   // Non-owning pointer. Guaranteed non-NULL after init is called.
   ProxyChannel::Delegate* delegate_;
 
+  // PID of the remote process. Use this instead of the Channel::peer_pid since
+  // this is set synchronously on construction rather than waiting on the
+  // "hello" message from the peer (which introduces a race condition).
+  base::ProcessId peer_pid_;
+
   // When we're unit testing, this will indicate the sink for the messages to
   // be deposited so they can be inspected by the test. When non-NULL, this
   // indicates that the channel should not be used.