Add a fast path to CreateCSSPropertyValueSet.

1 and 2 are the most common sizes of the vector passed into this
function, so we can add some simpler code for those cases. (The code
also trivially handles 0, which isn't as popular.)

Style perftest (Zen 2, LTO but no PGO):

Parse (µs)             Before     After    Perf      95% CI (BCa)
=================== ========= ========= ======= =================
ECommerce                1424      1418   +0.4%  [ +0.2%,  +0.7%]
Encyclopedia             7986      7900   +1.1%  [ +0.9%,  +1.3%]
Extension                1385      1377   +0.6%  [ -0.3%,  +1.2%]
News                     8509      8441   +0.8%  [ +0.7%,  +1.0%]
Search                   5379      5310   +1.3%  [ +0.8%,  +1.6%]
Social1                 15996     15440   +3.6%  [ +3.4%,  +3.8%]
Social2                   654       639   +2.4%  [ +2.0%,  +2.8%]
Sports                  61468     60853   +1.0%  [ +0.8%,  +1.2%]
Video                   32785     32486   +0.9%  [ +0.6%,  +1.7%]
Geometric mean                            +1.3%  [ +1.2%,  +1.5%]

Change-Id: Iec7d4be615d6c108c72d1a97d36792d9b586532b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5355752
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1270880}