[iOS] Fix creating window AX ID and using it at EG matcher.

Set the accessibility identifier at SceneDelegate when initializing
|_window|, instead of going through |self.sceneState|. Also used
|grey_accessibilityID| for matcher of this ID.

Bug: 1060786
Change-Id: I2b857d68e535087d6daa38c0656c0eff9f0abc01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2538138Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828896}

ios/chrome/browser/ui/main/scene_delegate.mm

@@ -44,7 +44,7 @@ NSString* const kOriginDetectedKey = @"OriginDetectedKey";
       // Assign an a11y identifier for using in EGTest.
       // See comment for [ChromeMatchersAppInterface windowWithNumber:] matcher
       // for context.
-      self.sceneState.window.accessibilityIdentifier =
+      _window.accessibilityIdentifier =
           [NSString stringWithFormat:@"%ld", UIApplication.sharedApplication
                                                      .connectedScenes.count -
                                                  1];

ios/chrome/test/earl_grey/chrome_matchers_app_interface.mm

@@ -130,7 +130,7 @@ UIView* SubviewWithAccessibilityIdentifier(NSString* accessibility_id,
 
 + (id<GREYMatcher>)windowWithNumber:(int)windowNumber {
   return grey_allOf(
-      grey_accessibilityLabel([NSString stringWithFormat:@"%d", windowNumber]),
+      grey_accessibilityID([NSString stringWithFormat:@"%d", windowNumber]),
       grey_kindOfClass([UIWindow class]), nil);
 }