Binds prefetching to the predictive networking privacy option.

Adds browser tests to check that both the privacy option and field
trial control prefetching.

BUG=334602

Review URL: https://codereview.chromium.org/142473003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247738 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247738