Change StyleVariables to an array-mapped trie.

Move StyleVariables from being two hash tables to a single array-mapped
trie (AMT) with copy-on-write. This is a more generic version of our
existing “root bucket optimization”, which works effieciently on any
number of levels, not just two. This allows us to handle more patterns
of setting custom properties without blowing up (although some variants
will still give us trouble, such as having a universal rule that apply
to a lot of leaves only).

The primary benefit of this change is robustness; I've added
a benchmark to blink_perf.css that becomes about 30x as fast under
this change.

The style perftest is neutral or slightly negative; some of the style
perftest subtests use ~40 kB more GCAlloc memory. Speedometer3 is
neutral or slightly positive (there's a bit too much noise to be sure).

Change-Id: I23b6f38719a3901e727ee769b9ca3de2588c417c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6367954
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Auto-Submit: Steinar H Gunderson <sesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1436784}