preload: Fix preloadingAttemptsOfPreviousPage in Devtools

This CL adds a flush in InspectorPageAgent::WillCommitLoad to
avoid a delay in CDP events, and this prevents the Devtools frontend
from getting outdated frame information.
The CL also updates Cross origin isolation tests to get COOP/COEP status
with synchronization(DidUpdatePolicyContainerHost). The flush makes `session.protocol.Network.getSecurityIsolationStatus` queries from the
renderer side can arrive earlier than before, and this makes the queries
arrive earlier than the update
`RenderFrameHostImpl::SetPolicyContainerHost` in the browser side.
See investigation document for more details
https://docs.google.com/document/d/1Ag43VrXuZf_lB7NgcSEVe6Au7O6sWMPAyh3EGYnPTYg/edit?usp=sharing

Bug: 331856214
Change-Id: I0d42519f88e3a24c18eec841c04589470be7d114
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5611590
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Huanpo Lin <robertlin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1316695}