Add template args for CRWWKScriptMessageRouter._handlers.

This CL  adds template args for CRWWKScriptMessageRouter._handlers to
make it more readable.

Bug: 956511
Change-Id: Iba9dfc5fb0068bb1e2966de5c40b667b1e53145e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649494Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667225}

ios/web/web_state/ui/crw_wk_script_message_router.mm

@@ -21,9 +21,11 @@
 @end
 
 @implementation CRWWKScriptMessageRouter {
-  // Two level map of registed message handlers. Keys are message names and
-  // values are more maps (where keys are web views and values are handlers).
-  NSMutableDictionary* _handlers;
+  // Two level map of registed message handlers:
+  //   {MessageName => {WKWebView => MessageCallbacks}}.
+  NSMutableDictionary<NSString*,
+                      NSMapTable<WKWebView*, void (^)(WKScriptMessage*)>*>*
+      _handlers;
   // Wrapped WKUserContentController.
   WKUserContentController* _userContentController;
 }