[Siri Shortcuts] Correctly check for searchPhrase existence

The previous way of checking whether the searchPhrase property existed
on the SearchInChrome intent object was respondsToSelector:, which
didn't work.

This fix uses valueForKey: instead, which appears to be a safe way to
check whether searchPhrase exists on the intent object.

Bug: 1133825
Change-Id: I82242e2398359f1955fd93b06ecdc4648fcaf5df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2459566
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815703}

ios/chrome/app/application_delegate/user_activity_handler.mm

@@ -170,10 +170,15 @@ + (BOOL)continueUserActivity:(NSUserActivity*)userActivity
         base::mac::ObjCCastStrict<SearchInChromeIntent>(
             userActivity.interaction.intent);
 
-    if (intent &&
-        [intent respondsToSelector:NSSelectorFromString(@"searchPhrase")] &&
-        intent.searchPhrase && [intent.searchPhrase length]) {
-      startupParams.textQuery = intent.searchPhrase;
+    if (!intent) {
+      return NO;
+    }
+
+    id searchPhrase = [intent valueForKey:@"searchPhrase"];
+
+    if ([searchPhrase isKindOfClass:[NSString class]] &&
+        [searchPhrase length]) {
+      startupParams.textQuery = searchPhrase;
     } else {
       startupParams.postOpeningAction = FOCUS_OMNIBOX;
     }