Fix a DCHECK failure in AppBannerManager when adding to homescreen from devtools.

chrome/browser/banners/app_banner_manager.cc

Index: chrome/browser/banners/app_banner_manager.cc
diff --git a/chrome/browser/banners/app_banner_manager.cc b/chrome/browser/banners/app_banner_manager.cc
index 03587cb6bdb9e00845a49354ec40a55e20f053df..a46c9dd012bfe2d0e632f0e29f8051c6811b7250 100644
--- a/chrome/browser/banners/app_banner_manager.cc
+++ b/chrome/browser/banners/app_banner_manager.cc
@@ -126,6 +126,9 @@ void AppBannerManager::RequestAppBanner(const GURL& validated_url,
     return;
   }
 
+  if (validated_url_.is_empty())
+    validated_url_ = validated_url;
+
   manager_->GetData(
       ParamsToGetManifest(),
       base::Bind(&AppBannerManager::OnDidGetManifest, GetWeakPtr()));
@@ -310,6 +313,7 @@ void AppBannerManager::Stop() {
   was_canceled_by_page_ = false;
   page_requested_prompt_ = false;
   need_to_log_status_ = false;
+  validated_url_ = GURL();
   referrer_.erase();
 }