Replace mini_installer's use of arraysize macro with _countof.
* Remove the duplication in the decompress.h
https://msdn.microsoft.com/en-us/library/ms175773.aspx
This issue arised when doing https://codereview.chromium.org/
1115993004/.
The original attempt was
https://chromium.googlesource.com/chromium/src/+/
016d71e2e048f0578e2c53c3f741755173ef8cb2,
but it was reverted in
https://chromium.googlesource.com/chromium/src/+/
4febac2867bb9dd87b033bdd1b059888c6faa9eb
because mini_installer can't use C++11 yet.
BUG=None
TEST=trybots
R=grt@chromium.org
Review URL: https://codereview.chromium.org/
1129953003
Cr-Commit-Position: refs/heads/master@{#328766}