ipc: Remove a CHECK to fix some memory tests.

The affected code isn't used in production yet, so this seems okay as a short
term solution to fix some tests. The problem is that --single_process and
similar flags are causing the supposedly process-wide singleton to be set from
different locations.

BUG=534539

Review URL: https://codereview.chromium.org/1360593003

Cr-Commit-Position: refs/heads/master@{#350045}

ipc/attachment_broker.cc

@@ -14,7 +14,10 @@ namespace IPC {
 
 // static
 void AttachmentBroker::SetGlobal(AttachmentBroker* broker) {
-  CHECK(!g_attachment_broker);
+  // TODO(erikchen): There should be a CHECK here to make sure that
+  // |g_attachment_broker| is nullptr. Right now, this causes problems with
+  // --single_process and similar testing conditions. I'm temporarily removing
+  // this CHECK. http://crbug.com/534539.
   g_attachment_broker = broker;
 }