Adds support for POST request with bodies on WKWebView.

ios/web/web_state/ui/crw_wk_script_message_router.mm

Index: ios/web/web_state/ui/crw_wk_script_message_router.mm
diff --git a/ios/web/web_state/ui/crw_wk_script_message_router.mm b/ios/web/web_state/ui/crw_wk_script_message_router.mm
index 5699abdfb92067217ee1189e988c8afada72153d..0e1dfbb33e0dd333119542563b4cbd246663c9b7 100644
--- a/ios/web/web_state/ui/crw_wk_script_message_router.mm
+++ b/ios/web/web_state/ui/crw_wk_script_message_router.mm
@@ -100,8 +100,12 @@
 - (void)tryRemoveScriptMessageHandlerForName:(NSString*)messageName
                                      webView:(WKWebView*)webView {
   NSMapTable* webViewToHandlerMap = [_handlers objectForKey:messageName];
-  if (![webViewToHandlerMap objectForKey:webView])
+  id handler = [webViewToHandlerMap objectForKey:webView];
+  if (!handler)
     return;
+  // Extend the lifetime of |handler| so removeScriptMessageHandlerForName: can
+  // be called from inside of |handler|.
+  [[handler retain] autorelease];
   if (webViewToHandlerMap.count == 1) {
     [_handlers removeObjectForKey:messageName];
     [_userContentController removeScriptMessageHandlerForName:messageName];