Add const-class benchmark.

Also fix indentation in const-string benchmark.

Test: vogar --benchmark art/benchmark/const-class/src/ConstClassBenchmark.java
Test: vogar --benchmark art/benchmark/const-string/src/ConstStringBenchmark.java
Bug: 30627598
Change-Id: I04ca9514f6e647640e2c0d9d65fdc4ace652927a
3 files changed