RFR: JDK-8297449: Update JInternalFrame Metal Border code

[Harshitha Onkar]

Updated Metal Border code for JInternalFrame. 

- Added instanceof check before casting Graphics to G2D.
- Replaced roundHalfDown with Region.clipRound()

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Commit messages:
 - removed unused import
 - JIF MetalBorder update

Changes: https://git.openjdk.org/jdk/pull/11305/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11305&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297449
  Stats: 112 lines in 1 file changed: 10 ins; 19 del; 83 mod
  Patch: https://git.openjdk.org/jdk/pull/11305.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11305/head:pull/11305

PR: https://git.openjdk.org/jdk/pull/11305