[TabModal] Short-term fix for JS dialog not showing in web apps

Use tab interactivity if tab model is not found in the tab model list.

Bug: 875595
Change-Id: I5fd69ed11e2f30733a25eaa98e353a98fc026a5e
Reviewed-on: https://chromium-review.googlesource.com/1188753Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Becky Zhou <huayinz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586187}

chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc

@@ -24,6 +24,7 @@
 #include "ui/gfx/text_elider.h"
 
 #if defined(OS_ANDROID)
+#include "chrome/browser/android/tab_android.h"
 #include "chrome/browser/ui/android/javascript_dialog_android.h"
 #include "chrome/browser/ui/android/tab_model/tab_model.h"
 #include "chrome/browser/ui/android/tab_model/tab_model_list.h"
@@ -40,8 +41,15 @@ app_modal::JavaScriptDialogManager* AppModalDialogManager() {
 bool IsWebContentsForemost(content::WebContents* web_contents) {
 #if defined(OS_ANDROID)
   TabModel* tab_model = TabModelList::GetTabModelForWebContents(web_contents);
-  return tab_model && tab_model->IsCurrentModel() &&
-         tab_model->GetActiveWebContents() == web_contents;
+  if (tab_model) {
+    return tab_model->IsCurrentModel() &&
+           tab_model->GetActiveWebContents() == web_contents;
+  } else {
+    // If tab model is not found (e.g. single tab model), fall back to check
+    // whether the tab for this web content is interactive.
+    TabAndroid* tab = TabAndroid::FromWebContents(web_contents);
+    return tab && tab->IsUserInteractable();
+  }
 #else
   Browser* browser = BrowserList::GetInstance()->GetLastActive();
   DCHECK(browser);