Traverse the sampled stack in the correct order. This results in several
order of magnitudes fewer flames rects.

- Fix flame rendering by having a fixed-width border
- Speed up flame rendering by setting shape-rendering to optimizeSpeed
- Fix rendering empty timelines

Bug: v8:10644, v8:11835
Change-Id: I5195d4d16a15c927ab25c7c111db69eeb0b0641a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2951728
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75084}