Previous calculation was off-by-one.

TBR=dnj@chromium.org, mmoss@chromium.org
BUG=598956

Review URL: https://codereview.chromium.org/1848793004 .

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299619 0039d316-1c4b-4281-b951-d872f2087c98