[safe-browsing] Cleanup tests and remove unnecessary prefix tracking.

[safe-browsing] Cleanup tests and remove unnecessary prefix tracking.

Shift PrefixSet::GetPrefixes() into private:, and make test users
friends appropriately.  This moved some tests into namespace
safe_browsing, removed duplicate safe_browsing:: in the PrefixSet
tests.

Modify manual chunk-loading code to use BeginChunk/FinishChunk around
logical chunks, and to not mix prefixes and full hashes in the same
chunk.  Remove cases where the tests were loading full hashes and the
corrosponding prefix, which was not representative of actual chunks
received from the server.

Remove the part of SBProcessSubs() which deleted full hashes by their
prefix component.  Since prefixes and full hashes cannot mix in a chunk,
this case should not be possible in the first place.

Fixed SBProcessSubsDeleteChunk to test what it claimed to test.

BUG=none

Review URL: https://codereview.chromium.org/263493002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267178 0039d316-1c4b-4281-b951-d872f2087c98