[Extensions] Mark ResultCatcher::GetNextResult() as WARN_UNUSED_RESULT

Callers of ResultCatcher::GetNextResult() should always, always check
the result. Typically, a return value of `false` means that something
went wrong (because it is returned when a test fails), so it should
be explicitly checked. (Note that we don't just assert in the
ResultCatcher itself, because some tests do expect to fail).

Update the one call site that didn't already check the result.

Bug: None
Change-Id: I690b794b0c1ac07cb0a30b5d3078ffdb258195a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1980752Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727473}

extensions/shell/browser/shell_desktop_controller_aura_browsertest.cc

@@ -111,7 +111,7 @@ IN_PROC_BROWSER_TEST_F(ShellDesktopControllerAuraBrowserTest, TwoAppWindows) {
             browser_context(), app_->id(),
             "chrome.app.window.create('/hello.html');"));
         ResultCatcher catcher;
-        catcher.GetNextResult();
+        ASSERT_TRUE(catcher.GetNextResult());
 
         // Close the first app window.
         GetAppWindow()->OnNativeClose();

extensions/test/result_catcher.h

@@ -31,7 +31,7 @@ class ResultCatcher : public content::NotificationObserver {
 
   // Pumps the UI loop until a notification is received that an API test
   // succeeded or failed. Returns true if the test succeeded, false otherwise.
-  bool GetNextResult();
+  bool GetNextResult() WARN_UNUSED_RESULT;
 
   void RestrictToBrowserContext(content::BrowserContext* context) {
     browser_context_restriction_ = context;