Remove calls to MessageLoop::current() in tools.

tools/gn/loader.cc

Index: tools/gn/loader.cc
diff --git a/tools/gn/loader.cc b/tools/gn/loader.cc
index 69fb7668e17dd200c23ec5b37a7f591c5f774d5a..5a3f7832e07acb589167fc4b526e8262e28afd3b 100644
--- a/tools/gn/loader.cc
+++ b/tools/gn/loader.cc
@@ -6,8 +6,7 @@
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
-#include "base/message_loop/message_loop.h"
-#include "base/single_thread_task_runner.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "tools/gn/build_settings.h"
 #include "tools/gn/err.h"
 #include "tools/gn/filesystem_utils.h"
@@ -100,9 +99,11 @@ SourceFile Loader::BuildFileForLabel(const Label& label) {
 // -----------------------------------------------------------------------------
 
 LoaderImpl::LoaderImpl(const BuildSettings* build_settings)
-    : main_loop_(base::MessageLoop::current()),
-      pending_loads_(0),
-      build_settings_(build_settings) {
+    : pending_loads_(0), build_settings_(build_settings) {
+  // There may not be an active TaskRunner at this point. When that's the case,
+  // the calling code is expected to call set_task_runner().
+  if (base::ThreadTaskRunnerHandle::IsSet())
+    task_runner_ = base::ThreadTaskRunnerHandle::Get();
 }
 
 LoaderImpl::~LoaderImpl() {
@@ -239,7 +240,7 @@ void LoaderImpl::BackgroundLoadFile(const Settings* settings,
                                     const LocationRange& origin,
                                     const ParseNode* root) {
   if (!root) {
-    main_loop_->task_runner()->PostTask(
+    task_runner_->PostTask(
         FROM_HERE, base::Bind(&LoaderImpl::DecrementPendingLoads, this));
     return;
   }
@@ -280,8 +281,7 @@ void LoaderImpl::BackgroundLoadFile(const Settings* settings,
 
   trace.Done();
 
-  main_loop_->task_runner()->PostTask(
-      FROM_HERE, base::Bind(&LoaderImpl::DidLoadFile, this));
+  task_runner_->PostTask(FROM_HERE, base::Bind(&LoaderImpl::DidLoadFile, this));
 }
 
 void LoaderImpl::BackgroundLoadBuildConfig(
@@ -289,7 +289,7 @@ void LoaderImpl::BackgroundLoadBuildConfig(
     const Scope::KeyValueMap& toolchain_overrides,
     const ParseNode* root) {
   if (!root) {
-    main_loop_->task_runner()->PostTask(
+    task_runner_->PostTask(
         FROM_HERE, base::Bind(&LoaderImpl::DecrementPendingLoads, this));
     return;
   }
@@ -339,9 +339,9 @@ void LoaderImpl::BackgroundLoadBuildConfig(
     }
   }
 
-  main_loop_->task_runner()->PostTask(
-      FROM_HERE, base::Bind(&LoaderImpl::DidLoadBuildConfig, this,
-                            settings->toolchain_label()));
+  task_runner_->PostTask(FROM_HERE,
+                         base::Bind(&LoaderImpl::DidLoadBuildConfig, this,
+                                    settings->toolchain_label()));
 }
 
 void LoaderImpl::DidLoadFile() {