[iOS Blink] Make AddNewContents method return a WebContent

As https://crrev.com/c/5791853 made AddNewContents() method
return a WebContents instance, ContentWebState class also
needs to return it. This CL changes it.

Bug: 359599878
Change-Id: If62507950099c67168b7aff9c26aa2dcee315ebb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5825951


Reviewed-by: Ali Juma <ajuma@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1349239}