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This CL fixes flaky timeout of external/wpt/first-image-child.html by:
* Adding a meta tag to let it take long.
* Adding HTML tags for iframe and img instead of adding the resources on
JS, to make it faster.
* Wrapping the core of the test under window load to ensure that the
resources are loaded by the time the entries are checked. It's possible
that before this CL the iframe was sometimes not ready to receive the
message from the parent, causing the test to timeout.

Bug: 1041709
Change-Id: I135c11f5f3a8a3f8e3f65012843a61bab7c50aa7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2025909
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736139}