RFR: 7083187: Class CSS.CssValue is missing implementations of equals() and hashCode()

[Prasanta Sadhukhan]

Two CSS AttributeSet-s can be compared using the AttributeSet.isEqual() method which can fail due to missing implementation of equals method in CSS subclasses.
In this issue, even when two CSS AttributeSet has same 42 font size string value, Object equality fails.
Fixed by implementing the equality and hashCode method for CSS.FontSize class.

All jtreg/jck tests are ok

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Commit messages:
 - Test fix
 - Test fix
 - 7083187: Class CSS.CssValue is missing implementations of equals() and hashCode()

Changes: https://git.openjdk.org/jdk/pull/13405/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13405&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-7083187
  Stats: 68 lines in 2 files changed: 67 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13405.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13405/head:pull/13405

PR: https://git.openjdk.org/jdk/pull/13405