RFR: 8318113: CSS.BackgroundImage doesn't implement equals

[Prasanta Sadhukhan]

CSSBackgroundImage.equals() is implemented

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Commit messages:
 - 8318113: CSS.BackgroundImage doesn't implement equals
 - 8318113: CSS.BackgroundImage doesn't implement equals

Changes: https://git.openjdk.org/jdk/pull/16613/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16613&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318113
  Stats: 15 lines in 2 files changed: 15 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16613.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16613/head:pull/16613

PR: https://git.openjdk.org/jdk/pull/16613