RFR: 8274238: Inconsistent type for young_list_target_length()

[brianjstafford]

This is a cleanup PR that changes the return type for G1Policy::young_list_target_length() to uint in order to be consistent with the underlying variable and all related variables and functions.

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Commit messages:
 - Changed return type for young_list_target_length() to uint

Changes: https://git.openjdk.java.net/jdk/pull/7357/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7357&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274238
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7357.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7357/head:pull/7357

PR: https://git.openjdk.java.net/jdk/pull/7357