RFR: 8276842 G1: Only calculate size in bytes from words when needed

[Hamlin Li]

This is a minor improvement in RemoveSelfForwardPtrObjClosure.
Currently, RemoveSelfForwardPtrObjClosure::do_object calculates from size in words to size in bytes every time, it's not necessary to do so, only needs to calculate the size when needed.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6305/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6305&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276842
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6305.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6305/head:pull/6305

PR: https://git.openjdk.java.net/jdk/pull/6305