Make MessageLoopProxy::current() usable on threads that don't have MessageLoop.

base/message_loop_proxy.h

Index: base/message_loop_proxy.h
diff --git a/base/message_loop_proxy.h b/base/message_loop_proxy.h
index 320ccfb079921e47d33d2b1c89f353960aefe56f..d0f3d27e7b7df4106b456eb419d72207d79b08c7 100644
--- a/base/message_loop_proxy.h
+++ b/base/message_loop_proxy.h
@@ -9,7 +9,7 @@
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
-#include "base/single_thread_task_runner.h"
+#include "base/thread_main_task_runner.h"
 
 namespace base {
 
@@ -23,7 +23,7 @@ namespace base {
 //
 // TODO(akalin): Now that we have the *TaskRunner interfaces, we can
 // merge this with MessageLoopProxyImpl.
-class BASE_EXPORT MessageLoopProxy : public SingleThreadTaskRunner {
+class BASE_EXPORT MessageLoopProxy : public ThreadMainTaskRunner {
  public:
   // Gets the MessageLoopProxy for the current message loop, creating one if
   // needed.