Enforce variations signature verification.

If a seed is received that fails verification, don't store it.
Similarly, on startup discard the seed if it fails verification.

Refactors some of the code to make tests pass (since they're
not testing signed data). Additionally, deprecates the previous
seed hashing code.

BUG=336536

Review URL: https://codereview.chromium.org/183003008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255697 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255697