[TabModal] Import JavaScript dialog tab helper code to Android

Create a dialog class that is managed by JavaScriptDialogTabHelper
for Android and show dummy "app modal" AlertDialog for now.

Bug: 687010
Change-Id: Iea8a5ff759f5975db4d67c1090082cfbe341c0bc
Reviewed-on: https://chromium-review.googlesource.com/702925
Commit-Queue: Becky Zhou <[email protected]>
Reviewed-by: Ted Choc <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#510002}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 7022b4bb..b0840cb 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -811,6 +811,9 @@
 extern const char kTabAudioMutingName[];
 extern const char kTabAudioMutingDescription[];
 
+extern const char kTabModalJsDialogName[];
+extern const char kTabModalJsDialogDescription[];
+
 extern const char kTabsInCbdName[];
 extern const char kTabsInCbdDescription[];