Rewrite std::string("") to std::string(), Linux edition.

content/browser/download/download_manager_impl_unittest.cc

Index: content/browser/download/download_manager_impl_unittest.cc
diff --git a/content/browser/download/download_manager_impl_unittest.cc b/content/browser/download/download_manager_impl_unittest.cc
index 4a331592a0cfba8f7e33661443713a122e4a3850..e08402e8ff8a051c36e0b36e1cffdff995a93b56 100644
--- a/content/browser/download/download_manager_impl_unittest.cc
+++ b/content/browser/download/download_manager_impl_unittest.cc
@@ -169,7 +169,9 @@ class MockDownloadItemImpl : public DownloadItemImpl {
   MOCK_CONST_METHOD0(GetUserVerifiedFilePath, base::FilePath());
   MOCK_METHOD0(NotifyRemoved, void());
   // May be called when vlog is on.
-  virtual std::string DebugString(bool verbose) const OVERRIDE { return ""; }
+  virtual std::string DebugString(bool verbose) const OVERRIDE {
+    return std::string();
+  }
 };
 
 class MockDownloadManagerDelegate : public DownloadManagerDelegate {