RFR: 8295323: Unnecessary HashTable usage in StyleSheet

[Andrey Turbanov]

Hashtable was used only from single thread. And only non-null key/values added to it. We can safely replace it with HashSet.

-------------

Commit messages:
 - [PATCH] Unnecessary HashTable usage in StyleSheet

Changes: https://git.openjdk.org/jdk/pull/10522/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10522&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295323
  Stats: 18 lines in 1 file changed: 0 ins; 2 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/10522.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10522/head:pull/10522

PR: https://git.openjdk.org/jdk/pull/10522