RFR: 8354791: Use Hashtable.putIfAbsent in CSS constructor [v2]

[Andrey Turbanov]

> We can use Hashtable.putIfAbsent instead of pair `get`/`put` methods.
> It's faster and cleaner.

Andrey Turbanov has updated the pull request incrementally with one additional commit since the last revision:

  add CSS. prefix

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Changes:
  - all: https://git.openjdk.org/jdk/pull/22032/files
  - new: https://git.openjdk.org/jdk/pull/22032/files/64cf26d9..5de8c8f7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22032&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22032&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22032.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22032/head:pull/22032

PR: https://git.openjdk.org/jdk/pull/22032