Ensure that in the SiteInstanceTest, no actual RenderProcessHosts are created.

BUG=none
TEST=tests keep passing

Review URL: https://codereview.chromium.org/72333003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235800 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235800