[Cleanup] Use WebContentsConsoleObserver in //chrome/browser/extensions

Update chrome/browser/extensions/process_manager_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: I887ead0ef81b5ea8b76dbf0696c48882a2fb48cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2327146Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792978}

chrome/browser/extensions/process_manager_browsertest.cc

@@ -1130,9 +1130,8 @@ IN_PROC_BROWSER_TEST_F(ProcessManagerBrowserTest,
         OpenPopup(main_frame, GURL(url::kAboutBlankURL));
     EXPECT_NE(popup, tab);
 
-    content::ConsoleObserverDelegate console_observer(
-        popup, "Not allowed to navigate top frame to*");
-    popup->SetDelegate(&console_observer);
+    content::WebContentsConsoleObserver console_observer(popup);
+    console_observer.SetPattern("Not allowed to navigate top frame to*");
     EXPECT_TRUE(ExecuteScript(
         popup, "location.href = '" + nested_urls[1].spec() + "';"));
     console_observer.Wait();