[safe-browsing] Database full hash matches like prefix match.
The safe-browsing code incorrectly replicates full-hashes into the
prefix set, and treats database full hashes as a form of cached
response. Move the database full hashes into PrefixSet in preparation
for removing the replication. Treat full-hash matches and prefix
matches the same (as matches which may lead to a gethash request).
ContainsBrowseUrl() full hash result changed to indicate that it is
previously-cached results.
Add a unit test that a full hash is persisted by PrefixSet. Right now
it succeeds because of the replication of full-hash prefixes into the
prefix set.
BUG=361248,172527
Review URL: https://codereview.chromium.org/
257383006
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267498 0039d316-1c4b-4281-b951-d872f2087c98