[M152] perf_ui: render popover contents with lit consistently in flamechart

Original change's description:
> perf_ui: render popover contents with lit consistently in flamechart
>
> Consistently use Lit render() for all updates to popoverElement in
> FlameChart.ts (including clearing via render(nothing, ...)).
>
> Previously, commit 09c57bf849 introduced Lit rendering in
> updatePopoverContents(), while other methods (hideHighlight() and
> #updatePopoverForGroup()) continued to imperatively clear or modify
> popoverElement via removeChildren() or innerText. Wiping Lit internal
> comment markers caused Lit to throw TypeError on subsequent calls.
>
> Bug: None
> Change-Id: Ib5c8f2b686e827bac69c26406cace3afd8c34fe4
> Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/8144819
> Commit-Queue: Jack Franklin <jacktfranklin@chromium.org>
> Auto-Submit: Paul Irish <paulirish@chromium.org>
> Reviewed-by: Jack Franklin <jacktfranklin@chromium.org>

(cherry picked from commit 39bf1ee77839f1789c0c6634a423f7be82b6760c)

Bug: 553885466,None
Change-Id: Ib5c8f2b686e827bac69c26406cace3afd8c34fe4
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/8315170
Reviewed-by: Paul Irish <paulirish@chromium.org>
Auto-Submit: chrome-cherry-picker@chops-service-accounts.iam.gserviceaccount.com <chrome-cherry-picker@chops-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Jack Franklin <jacktfranklin@chromium.org>
Reviewed-by: Jack Franklin <jacktfranklin@chromium.org>