The number of cookies deleted will always be zero or a positive number,
so should be represented by an unsigned.  This is being done to bring
the CookieStore interface into alignment with the Mojo cookie service.

BUG=721395
R=mmenke@chromium.org
TBR=sgurun@chromium.org
TBR=mkwst@chromium.org
TBR=sky@chromium.org
TBR=jam@chromium.org
TBR=rohitrao@chromium.org

Review-Url: https://codereview.chromium.org/2971733002
Cr-Commit-Position: refs/heads/master@{#485148}