content: Add an overridable task runner to ChildThreadImpl

content/child/worker_thread_message_filter.cc

Index: content/child/worker_thread_message_filter.cc
diff --git a/content/child/worker_thread_message_filter.cc b/content/child/worker_thread_message_filter.cc
index aa11b3bb70757b900f6d573194a3b3d5289b608f..78df826f05cd1004088a381cc59c197776078f8e 100644
--- a/content/child/worker_thread_message_filter.cc
+++ b/content/child/worker_thread_message_filter.cc
@@ -5,6 +5,7 @@
 #include "content/child/worker_thread_message_filter.h"
 
 #include "base/thread_task_runner_handle.h"
+#include "content/child/child_thread_impl.h"
 #include "content/child/thread_safe_sender.h"
 #include "content/child/worker_task_runner.h"
 #include "ipc/ipc_message_macros.h"
@@ -13,7 +14,7 @@ namespace content {
 
 WorkerThreadMessageFilter::WorkerThreadMessageFilter(
     ThreadSafeSender* thread_safe_sender)
-    : main_thread_task_runner_(base::ThreadTaskRunnerHandle::Get()),
+    : main_thread_task_runner_(ChildThreadImpl::Get()->GetTaskRunner()),
       thread_safe_sender_(thread_safe_sender) {
 }