Move ReadFileToString to the base namespace.

ash/desktop_background/desktop_background_controller.cc

Index: ash/desktop_background/desktop_background_controller.cc
diff --git a/ash/desktop_background/desktop_background_controller.cc b/ash/desktop_background/desktop_background_controller.cc
index 267d962fa5061101db1c8d2aacdd0f1dc60eefd2..1456defe3599a95c5024670ee1e9c56fa7ba06d0 100644
--- a/ash/desktop_background/desktop_background_controller.cc
+++ b/ash/desktop_background/desktop_background_controller.cc
@@ -115,7 +115,7 @@ class DesktopBackgroundController::WallpaperLoader
   static scoped_ptr<SkBitmap> LoadSkBitmapFromJPEGFile(
       const base::FilePath& path) {
     std::string data;
-    if (!file_util::ReadFileToString(path, &data)) {
+    if (!base::ReadFileToString(path, &data)) {
       LOG(ERROR) << "Unable to read data from " << path.value();
       return scoped_ptr<SkBitmap>();
     }