[iOS] Enable multiwindow on trunk by default

This CL enables iPad multi-window on trunk by default.

Change-Id: I919f62fc8f0068071358a9148ba0533a9abedbfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404929
Auto-Submit: Mark Cogan <marq@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806553}

ios/build/chrome_build.gni

@@ -27,7 +27,7 @@ declare_args() {
 
   # Enable multi-window configuration in Info.plist. Includes all the effects of
   # setting |ios_enable_scene_startup| to |true|.
-  ios_enable_multi_window = false
+  ios_enable_multi_window = true
 
   # Value of the encryption export compliance code. See "Cryptography and
   # U.S. Export Compliance" in "Submitting the App to App Review" in the