Revert of [gn] Experiment: Reset -O3 to -O2. (patchset #1 id:1 of https://codereview.chromium.org/2135313002/ )

Reason for revert:
Data collected

Original issue's description:
> [gn] Experiment: Reset -O3 to -O2.
>
> Temporary commit to see performance data. Will be reverted
> shortly after.
>
> BUG=chromium:622332
> TBR=vogelheim, jochen
> NOTRY=true
>
> Committed: https://crrev.com/bbb61d8aea732457e1d5a0c9abdd79eacf1622ae
> Cr-Commit-Position: refs/heads/master@{#37667}

TBR=vogelheim@chromium.org,jochen@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:622332

Review-Url: https://codereview.chromium.org/2141113002
Cr-Commit-Position: refs/heads/master@{#37690}