Add JavaScript prompts support to CWVUIDelegate.

ios/web_view/public/cwv_ui_delegate.h

Index: ios/web_view/public/cwv_ui_delegate.h
diff --git a/ios/web_view/public/cwv_ui_delegate.h b/ios/web_view/public/cwv_ui_delegate.h
index 6aa3cffd206c7f9aea652f16fe0e6d6e8e25a179..9baeb01bbe9fa5726d97f50583cce3c09dfc825d 100644
--- a/ios/web_view/public/cwv_ui_delegate.h
+++ b/ios/web_view/public/cwv_ui_delegate.h
@@ -25,6 +25,29 @@
                      inView:(UIView*)view
         userGestureLocation:(CGPoint)location;
 
+// Instructs the delegate to show UI in response to window.alert JavaScript
+// call.
+- (void)webView:(CWVWebView*)webView
+    runJavaScriptAlertPanelWithMessage:(NSString*)message
+                               pageURL:(NSURL*)URL
+                     completionHandler:(void (^)(void))completionHandler;
+
+// Instructs the delegate to show UI in response to

[Eugene But (OOO till 7-30), 2017/02/28 01:18:58]

nit:

// Instructs the delegate to show UI in response to window.confirm JavaScript
// call.

[michaeldo, 2017/02/28 06:10:38]

Done.

+// window.confirm JavaScript call.
+- (void)webView:(CWVWebView*)webView
+    runJavaScriptConfirmPanelWithMessage:(NSString*)message
+                                 pageURL:(NSURL*)URL
+                       completionHandler:(void (^)(BOOL))completionHandler;
+
+// Instructs the delegate to show UI in response to window.prompt

[Eugene But (OOO till 7-30), 2017/02/28 01:18:58]

nit: 

// Instructs the delegate to show UI in response to window.prompt JavaScript
// call.

[michaeldo, 2017/02/28 06:10:38]

Done.

+// JavaScript call.
+- (void)webView:(CWVWebView*)webView
+    runJavaScriptTextInputPanelWithPrompt:(NSString*)prompt
+                              defaultText:(NSString*)defaultText
+                                  pageURL:(NSURL*)URL
+                        completionHandler:
+                            (void (^)(NSString*))completionHandler;
+
 @end
 
 #endif  // IOS_WEB_VIEW_PUBLIC_CWV_UI_DELEGATE_H_