Switch AppBannerManager on use of Unretained to use it's weakptr factory.

chrome/browser/android/banners/app_banner_manager.cc

Index: chrome/browser/android/banners/app_banner_manager.cc
diff --git a/chrome/browser/android/banners/app_banner_manager.cc b/chrome/browser/android/banners/app_banner_manager.cc
index 56a954143f32a26ecbb06cbff117773e610d10ed..8da89910b030580d004918cecc061be37c969c67 100644
--- a/chrome/browser/android/banners/app_banner_manager.cc
+++ b/chrome/browser/android/banners/app_banner_manager.cc
@@ -138,11 +138,9 @@ void AppBannerManager::DidFinishLoad(
   if (!validated_url_.SchemeIsSecure() && !gDisableSecureCheckForTesting)
     return;
 
-  // See if the page has a manifest. Using Unretained(this) here is safe as the
-  // lifetime of this object extends beyond the lifetime of the web_contents(),
-  // and when web_contents() is destroyed it will call OnDidGetManifest.
+  // See if the page has a manifest.
   web_contents()->GetManifest(base::Bind(&AppBannerManager::OnDidGetManifest,
-                                         base::Unretained(this)));
+                                         weak_factory_.GetWeakPtr()));
 }
 
 void AppBannerManager::OnDidGetManifest(const content::Manifest& manifest) {