[ios] Add TODO to record usages of "return std::move(foo)" pattern.

ios/chrome/browser/browser_state/test_chrome_browser_state.mm

Index: ios/chrome/browser/browser_state/test_chrome_browser_state.mm
diff --git a/ios/chrome/browser/browser_state/test_chrome_browser_state.mm b/ios/chrome/browser/browser_state/test_chrome_browser_state.mm
index 4c4e4d86973fd09df648e4b1411f0cd912705a0c..be29b43451a8bc8556daa369fc4f822dd30ce44c 100644
--- a/ios/chrome/browser/browser_state/test_chrome_browser_state.mm
+++ b/ios/chrome/browser/browser_state/test_chrome_browser_state.mm
@@ -72,6 +72,7 @@ std::unique_ptr<KeyedService> BuildBookmarkModel(web::BrowserState* context) {
       browser_state->GetPrefs(),
       browser_state->GetStatePath(), browser_state->GetIOTaskRunner(),
       web::WebThread::GetTaskRunnerForThread(web::WebThread::UI));
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(bookmark_model);
 }