Add plumbing for persisting a signature for the variations seed.

In a later CL, I will add logic to actually use this signature
for validating the integrity of the seed.

BUG=336536

Review URL: https://codereview.chromium.org/143803008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246702 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246702
Browser Plugin: Perform NULL Check before grabbing embedder's RenderWidgetHostView.

BUG=336330

Review URL: https://codereview.chromium.org/141753013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246703 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246703