Prerender: Deflake iframe test with sequential prerendering

This CL deflakes the disabled iframe test with the sequential
prerendering feature.

The iframe test was failing because it attempted to insert an iframe
into the main frame that is still in the middle of the initial
prerendering navigation. Calling `WaitForPrerenderLoadCompletion`
before the insertion will resolve this test failing issue.

Bug: 1355151, 1365781
Change-Id: Ibead81c63c4c73dfc645f5316c76c269712e212e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3907381
Commit-Queue: Yoshiki Tanioka <tanioka@google.com>
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1049439}