Don't force layout at parse completion of subframe if it is hidden

This call is not needed, and might be causing
performance issues in cases where multiple hidden iframes are added
(which is a common case with 3p libraries like GTM).

The test asserts that performing a style-affecting change at the load
event, which would occur right before that forced layout, would apply
only in the next render frame and wouldn't add another layout operation.

Bug: 324572951
Change-Id: I8739c25a869c08515d49866eeba4f63e38fc54f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6733102
Commit-Queue: Noam Rosenthal <nrosenthal@google.com>
Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1494397}