Reland of Delete cryptstring.h and cryptstring.cc. (patchset #1 id:1 of https://codereview.webrtc.org/2742743002/ )

Reason for revert:
CryptString usage is now deleted in Chrome, see cl https://codereview.chromium.org/2738973004/

Original issue's description:
> Revert of Delete cryptstring.h and cryptstring.cc. (patchset #1 id:1 of https://codereview.webrtc.org/2740633003/ )
>
> Reason for revert:
> It turns out cryptstring was used by Chrome. In the xmpp code recently moved there from webrtc.
>
> So this code has to be moved too, it canät just be deleted.
>
> Original issue's description:
> > Delete cryptstring.h and cryptstring.cc.
> >
> > They became unused with cl https://codereview.webrtc.org/2731673002/
> >
> > BUG=webrtc:6424
> >
> > Review-Url: https://codereview.webrtc.org/2740633003
> > Cr-Commit-Position: refs/heads/master@{#17128}
> > Committed: 822638b481
>
> TBR=pthatcher@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:6424
>
> Review-Url: https://codereview.webrtc.org/2742743002
> Cr-Commit-Position: refs/heads/master@{#17130}
> Committed: d665207601

TBR=pthatcher@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:6424

Review-Url: https://codereview.webrtc.org/2745523004
Cr-Commit-Position: refs/heads/master@{#17160}

webrtc/base/socketadapters.h

@@ -16,7 +16,6 @@
 
 #include "webrtc/base/asyncsocket.h"
 #include "webrtc/base/constructormagic.h"
-#include "webrtc/base/cryptstring.h"
 #include "webrtc/base/logging.h"
 
 namespace rtc {