[Cleanup] Use WebContentsConsoleObserver in content/browser/download/

Update //content/browser/download/mhtml_generation_browsertest.cc to
use WebContentsConsoleObserver instead of ConsoleObserverDelegate.
This allows us to observe console messages added without overriding
the existing delegate of the WebContents.

Bug: 1034150
Change-Id: I79d7d6b8174e5e24c4a9ad776a94e2636ec6a9b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2327287Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793659}

content/browser/download/mhtml_generation_browsertest.cc

@@ -359,18 +359,14 @@ class MHTMLGenerationTest
       return;
 
     // Loads the generated file to check if it is well formed.
-    WebContentsDelegate* old_delegate = shell()->web_contents()->GetDelegate();
-    ConsoleObserverDelegate console_delegate(shell()->web_contents(),
-                                             "Malformed multipart archive: *");
-    shell()->web_contents()->SetDelegate(&console_delegate);
+    WebContentsConsoleObserver console_observer(shell()->web_contents());
+    console_observer.SetPattern("Malformed multipart archive: *");
 
     EXPECT_TRUE(
         NavigateToURL(shell(), net::FilePathToFileURL(params.file_path)))
         << "Error navigating to the generated MHTML file";
-    EXPECT_EQ(0U, console_delegate.message().length())
+    EXPECT_TRUE(console_observer.messages().empty())
         << "The generated MHTML file is malformed";
-
-    shell()->web_contents()->SetDelegate(old_delegate);
   }
 
   void TwoStepSyncTestFor(const TaskOrder order);