[HBASE-9815] Fixing TestFlashbackQuery and TestHeapSize.

Author: manukranthk

Summary: The allocate should be called before returning the histogram. For TestHeapSize, the number of references in the Store is increased by 1 so, FIXED_OVERHEAD needs to be bumped up by 1.

Test Plan: Ensuring the unit tests pass.

Reviewers: adela, liyintang, daviddeng

Reviewed By: adela

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D1078262

Task ID: 3297578

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1548982 13f79535-47bb-0310-9956-ffa450edef68