content: Add an overridable task runner to ChildThreadImpl

content/public/child/child_thread.h

Index: content/public/child/child_thread.h
diff --git a/content/public/child/child_thread.h b/content/public/child/child_thread.h
index 6293ed164d8443dbd474ce35934a3235c0337943..fc931b9f3810aa3073b0087c59a62e73aaf7cd9c 100644
--- a/content/public/child/child_thread.h
+++ b/content/public/child/child_thread.h
@@ -5,6 +5,7 @@
 #ifndef CONTENT_PUBLIC_CHILD_CHILD_THREAD_H_
 #define CONTENT_PUBLIC_CHILD_CHILD_THREAD_H_
 
+#include "base/memory/ref_counted.h"
 #include "content/common/content_export.h"
 #include "ipc/ipc_sender.h"
 
@@ -12,6 +13,10 @@
 #include <windows.h>
 #endif
 
+namespace base {
+class SingleThreadTaskRunner;
+}
+
 namespace content {
 
 class CONTENT_EXPORT ChildThread : public IPC::Sender {
@@ -22,6 +27,12 @@ class CONTENT_EXPORT ChildThread : public IPC::Sender {
 
   ~ChildThread() override {}
 
+  // Returns the task runner which should be used to post tasks to be executed
+  // on this child thread. Note that this may be different from the underlying
+  // message loop for this thread, and should therefore be used instead of
+  // going to the message loop directly.
+  virtual scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner() = 0;
+
 #if defined(OS_WIN)
   // Request that the given font be loaded by the browser so it's cached by the
   // OS. Please see ChildProcessHost::PreCacheFont for details.