Fix CompactionPickerTest.Level1Trigger2

Summary: CompactionPickerTest.Level1Trigger2 now depends on the STL implementation to be correct. Fix it.

Test Plan: Run the test

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D27963
Fix build break because of unsigned/signed mismatch
Merge pull request #371 from dlezama/master

Fix build break because of unsigned/signed mismatch