RFR: 8275150: URLClassLoaderTable should store OopHandle instead of Handle

[Calvin Cheung]

Please review this small patch for storing the URLClassLoader as OopHandle instead of Handle.
It is to avoid running into the same issue as in JDK-8274753.

Passed tiers 1 and 2 tests.

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Commit messages:
 - 8275150: URLClassLoaderTable should store OopHandle instead of Handle

Changes: https://git.openjdk.java.net/jdk/pull/5972/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5972&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275150
  Stats: 6 lines in 1 file changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5972.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5972/head:pull/5972

PR: https://git.openjdk.java.net/jdk/pull/5972