Adding lsan root unregistration when Free-ing VirtualMemory.

Bug: chromium:787976
Change-Id: I0ef32b4324ff6d7b6192d7e5616081766a0e51a8
Reviewed-on: https://chromium-review.googlesource.com/786995Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49609}