[ios] Fix Main Intent Metric logging

IOSLaunchedBySearchInChromeIntent is being logged instead of
IOSOpenByMainIntent. This change fixes that.

Bug: 1154422
Change-Id: If77133adeceb18296e7fbc1f284c13348923793d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2568650
Auto-Submit: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832574}

ios/chrome/app/main_application_delegate.mm

@@ -324,7 +324,7 @@ const int kMainIntentCheckDelay = 1;
           }
           if (!appStartupFromExternalIntent) {
             base::RecordAction(
-                base::UserMetricsAction("IOSLaunchedBySearchInChromeIntent"));
+                base::UserMetricsAction("IOSOpenByMainIntent"));
           }
 
           [_appState