Revert 259199 "Move DNS API to //extensions."

Revert 259199 "Move DNS API to //extensions."

Reverting because this probably broke browser_tests on Windows:
http://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%283%29/builds/27366/steps/browser_tests/logs/MiddleClickTargetBlankOrigin
http://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%28dbg%29%283%29/builds/25769/steps/browser_tests/logs/URLLoader3
http://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%28dbg%29%283%29/builds/25769/steps/browser_tests/logs/VerifyEncryption

Looks like there is something wrong with the changes around contexts / threads.

> Move DNS API to //extensions.
>
> This was an easy target to move and it kills one more //chrome dep
> from //extensions.
>
> BUG=355752
> TBR=rpaquay@chromium.org FYI for mechanical sockets changes
>
> Review URL: https://codereview.chromium.org/209473012

TBR=rockot@chromium.org

Review URL: https://codereview.chromium.org/210253005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@259263 0039d316-1c4b-4281-b951-d872f2087c98