Add stub of persistent_histogram_allocator.h

This is to support Chromium UMA integration in crashpad_handler as per
https://codereview.chromium.org/2308763002/.

R=mark@chromium.org
BUG=crashpad:100

Change-Id: I89eb8e835dabf210160d64ad47231b8c57c5fed6
Reviewed-on: https://chromium-review.googlesource.com/386236
Reviewed-by: Mark Mentovai <mark@chromium.org>