Reland "Reapply "Headless: don't run unload hooks upon web contents close.""

This reverts commit 03de68f885736c9da097acabdf853d0c66fd2759.

Reason for revert: Need more logs

Original change's description:
> Revert "Reapply "Headless: don't run unload hooks upon web contents close.""
> 
> This reverts commit dc9251bda9e350c261d9cf69380d1e3244962192.
> 
> Reason for revert: headless_browsertests are flaky on Win7:
> https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Win7%20Tests%20%28dbg%29%281%29. This looks like the only possible culprit.
> 
> Original change's description:
> > Reapply "Headless: don't run unload hooks upon web contents close."
> > 
> > Change-Id: Ic98de1070c3cee34a926ee0a95eb05cd62345ce6
> > Reviewed-on: https://chromium-review.googlesource.com/1113309
> > Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
> > Commit-Queue: Pavel Feldman <pfeldman@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#570003}
> 
> TBR=caseq@chromium.org,pfeldman@chromium.org
> 
> # Not skipping CQ checks because original CL landed > 1 day ago.
> 
> Change-Id: Idb3d74d77c445ee6e870ca3bf476e8a34374b0f9
> Reviewed-on: https://chromium-review.googlesource.com/1115839
> Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
> Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#570588}

TBR=dgozman@chromium.org,caseq@chromium.org,pfeldman@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: I2e043add07df3c89dbd5a833acbd407c972d4cfc
Reviewed-on: https://chromium-review.googlesource.com/1128107
Commit-Queue: Pavel Feldman <pfeldman@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573168}
3 files changed