Deflake a largest contentful paint in Iframe browser test

The test was flaky because the test generates a largest contentful
paint, in a iframe and they should be sent from renderer to browser, but
we always do not receive the LCP value before we start to check the
metrics in the test. We need to add the largest contentful paint waiter
to make sure we have received the LCP value in the browser side before
we start check the value of metrics in the test.

Bug: 1288027
Change-Id: Ie6212b4e9e763ee04f5690525a9cb8516c54d2d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3632974
Reviewed-by: Nicolás Peña <npm@chromium.org>
Commit-Queue: Lan Wei <lanwei@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1001542}