Remove IPC::BrokerableAttachment.

With only ChannelMojo in use, the distinction between brokerable and
non-brokerable attachments no longer makes sense. This CL removes that
distinction by removing BrokerableAttachment and flattening the
hierarchy of attachment types.

This also trims some POSIX-specific parts of IPC::MessageAttachmentSet.

BUG=659448

Committed: https://crrev.com/f6e03ce56c4d2370b79d0c3dd4ceb89cf5528e56
Review-Url: https://codereview.chromium.org/2494943002
Cr-Original-Commit-Position: refs/heads/master@{#432153}
Cr-Commit-Position: refs/heads/master@{#434099}
diff --git a/ipc/ipc_send_fds_test.cc b/ipc/ipc_send_fds_test.cc
index f648b64..3c814bb 100644
--- a/ipc/ipc_send_fds_test.cc
+++ b/ipc/ipc_send_fds_test.cc
@@ -77,8 +77,8 @@
         num_fds_received_(0) {
   }
 
-  bool GotExpectedNumberOfDescriptors() const {
-    return num_fds_received_ == kNumFDsToSend * kNumMessages;
+  unsigned num_fds_received() const {
+    return num_fds_received_;
   }
 
   void OnChannelError() override {
@@ -163,7 +163,7 @@
 
     // Verify that the message loop was exited due to getting the correct number
     // of descriptors, and not because of the channel closing unexpectedly.
-    EXPECT_TRUE(listener.GotExpectedNumberOfDescriptors());
+    EXPECT_EQ(kNumFDsToSend * kNumMessages, listener.num_fds_received());
 
     Close();
   }