Fix PrefixSet valgrind error, take#2.

Recent CL r253548 "Revert safe-browsing PrefixSet size_t handling." reverted
r249644, which was no longer needed.  But it also reverted r250505 "Fix
uninitialized memory read in PrefixSet test."

Fix that.

BUG=342903
TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/182133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253592 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253592