Safe-browsing PrefixSet cleanups.

Make sure SBPrefix is a fixed size.
PrefixSet tests for single-element set, set with large deltas, and
  int32 space edge cases.
PrefixSet::GetPrefixes() can be const.
Consolidate the SafeBrowsingDatabase GetPrefixes() checking code.
Check whether deltas fit by directly checking whether the delta fit.
Add a histogram for checking if SBPrefix really was crazy.

BUG=71832
TEST=none

Review URL: http://codereview.chromium.org/6711021

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@78667 0039d316-1c4b-4281-b951-d872f2087c98
5 files changed