Assume non-null URI from Intent to prevent invalid usage

In createCustomTabActivityIntent(), IntentHandler.getUrlFromIntent() may
return null, but Uri.parse() does not accept a null parameter. This CL
adds an assumeNonNull check to ensure the method is not called with
invalid input and to avoid a potential NullPointerException crash.

Change-Id: Id97bdc82e3ae70e2bcad564c557966f6b9f877df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7267445
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: Jingping Sun <jings@microsoft.com>
Commit-Queue: Jingping Sun <jings@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1565477}