Update tab_usage_recorder_test_util to use warn_unused_result.

This is in line with other helper functions which had GREYAssert
converted to NSError as part of the EG2 conversion.

Bug: 922813
Change-Id: Ib4e44af9a45555733e2773ef40b55df2cf79b97f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566456Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: ericale <ericale@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650470}

ios/chrome/browser/metrics/tab_usage_recorder_test_util.h

@@ -5,16 +5,18 @@
 #ifndef IOS_CHROME_BROWSER_METRICS_TAB_USAGE_RECORDER_TEST_UTIL_H_
 #define IOS_CHROME_BROWSER_METRICS_TAB_USAGE_RECORDER_TEST_UTIL_H_
 
+#include "base/compiler_specific.h"
+
 @class NSError;
 
 namespace tab_usage_recorder_test_util {
 
 // Opens a new incognito tab using the UI and evicts any main tab model tabs.
-NSError* OpenNewIncognitoTabUsingUIAndEvictMainTabs();
+NSError* OpenNewIncognitoTabUsingUIAndEvictMainTabs() WARN_UNUSED_RESULT;
 
 // Switches to normal mode using the tab switcher and selects the
 // previously-selected normal tab. Assumes current mode is Incognito.
-NSError* SwitchToNormalMode();
+NSError* SwitchToNormalMode() WARN_UNUSED_RESULT;
 
 }  // namespace tab_usage_recorder_test_util