This reverts commit 2963f1b2.

Reason for revert: Speculative revert, possibly causing timeout in layout test https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/WebKit%20Linux%20Trusty%20MSAN/11631

Original change's description:
> [parser] Simplify ParseFunctionBody
> 
> - Merge is_simple branches at the top
> - Remove block around inner_body parsing. Always merge fully at the end.
> - Remove conditional inner block adding to outer body. Simply add it to the
>   inner body making merge push it to the parent.
> 
> Change-Id: I1f062918a7abac354b949136463517bd0440984f
> Reviewed-on: https://chromium-review.googlesource.com/c/1386111
> Commit-Queue: Toon Verwaest <verwaest@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#58390}

TBR=ishell@chromium.org,verwaest@chromium.org

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

Change-Id: I0ab903df5caaadff56625e9b46b03981c8a9b930
Reviewed-on: https://chromium-review.googlesource.com/c/1388524Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58436}