RFR: JDK-8294484: MetalBorder's FrameBorder & DialogBorder have border rendering issues when scaled [v3]

[Harshitha Onkar]

> FrameBorder and DialogBorder had border scaling issues similar to JInternalFrame. This fix addresses it by creating `AbstractMetalBorder` class within MetalBorder, that contains the common steps required for painting border for `FrameBorder`, `DialogBorder` and `InternalFrameBorder`. 
> 
> All 3 cases - JFrame, JDialog and JInternalFrame are combined into a single test case - `ScaledMetalBorderTest` and hence the older `InternalFrameBorderTest` which is no longer required, is deleted.

Harshitha Onkar has updated the pull request incrementally with one additional commit since the last revision:

  minor change

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12391/files
  - new: https://git.openjdk.org/jdk/pull/12391/files/a0e8b29e..ab1e6e79

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12391&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12391&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12391.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12391/head:pull/12391

PR: https://git.openjdk.org/jdk/pull/12391