Wait for first paint in lens overlay webui tests.

Since we run these tests on the overlay contents which depends on the
screenshot, we should wait for the first paint on the active tab to
prevent flakiness.

Change-Id: I1ec1608ce217a56611017c2869ac66e9b56cdfa4
Bug: b:332393615
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5411316


Commit-Queue: Juan Mojica <juanmojica@google.com>
Auto-Submit: Juan Mojica <juanmojica@google.com>
Reviewed-by: Duncan Mercer <mercerd@google.com>
Commit-Queue: Duncan Mercer <mercerd@google.com>
Cr-Commit-Position: refs/heads/main@{#1280937}