Add public API for handling Javascript alerts and prompt.

ios/web/public/web_state/web_state_delegate.h

Index: ios/web/public/web_state/web_state_delegate.h
diff --git a/ios/web/public/web_state/web_state_delegate.h b/ios/web/public/web_state/web_state_delegate.h
index bebe64ea8f2772c5c0b20606d9f05505decda7f5..c7e8e35518e5a6434f6366c36b3de6e339aae167 100644
--- a/ios/web/public/web_state/web_state_delegate.h
+++ b/ios/web/public/web_state/web_state_delegate.h
@@ -16,6 +16,7 @@ class SSLInfo;
 namespace web {
 
 struct ContextMenuParams;
+class DialogPresenter;
 struct SSLStatus;
 class WebState;
 
@@ -36,6 +37,12 @@ class WebStateDelegate {
   virtual bool HandleContextMenu(WebState* source,
                                  const ContextMenuParams& params);
 
+  // Returns a pointer to a service to manage dialogs. May return nullptr in
+  // which case dialogs aren't shown.
+  // TODO(crbug.com/622084): Move this out of WebStateDelegate and into
+  // WebDependencyFactory along with the analogous method in content//.

[Eugene But (OOO till 7-30), 2016/06/24 17:04:52]

NIT: We don't know if WebDependencyFactory will exist, please use more generic
comment, like "Find better place for this method."

[michaeldo, 2016/06/27 20:30:10]

Done.

+  virtual DialogPresenter* GetDialogPresenter(WebState* source);
+
  protected:
   virtual ~WebStateDelegate();