base: Convert the remaining uses of MessageLoop::RunUntilIdle to RunLoop variant.

base/files/important_file_writer_unittest.cc

Index: base/files/important_file_writer_unittest.cc
diff --git a/base/files/important_file_writer_unittest.cc b/base/files/important_file_writer_unittest.cc
index 970e9f61d38275542b1a7dad0472bea745e86d8a..65da8146dcc7e2a39dada04180d4ee04477bdf37 100644
--- a/base/files/important_file_writer_unittest.cc
+++ b/base/files/important_file_writer_unittest.cc
@@ -10,6 +10,7 @@
 #include "base/files/scoped_temp_dir.h"
 #include "base/logging.h"
 #include "base/message_loop.h"
+#include "base/run_loop.h"
 #include "base/threading/thread.h"
 #include "base/time.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -63,7 +64,7 @@ TEST_F(ImportantFileWriterTest, Basic) {
                              MessageLoopProxy::current());
   EXPECT_FALSE(file_util::PathExists(writer.path()));
   writer.WriteNow("foo");
-  loop_.RunUntilIdle();
+  RunLoop().RunUntilIdle();
 
   ASSERT_TRUE(file_util::PathExists(writer.path()));
   EXPECT_EQ("foo", GetFileContent(writer.path()));