Revert of Remove user-defined move constructor and assignment operator from WTF::String

Revert of Remove user-defined move constructor and assignment operator from WTF::String (https://codereview.chromium.org/367343003/)

Reason for revert:
This can be applied only when user-defined dtor is removed.

Original issue's description:
> Remove user-defined move constructor and assignment operator from WTF::String
> 
> WTF::String can rely on implicit move constructor and assignment
> operator as WTF::RefPtr class has them defined now (r177066).
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178541

TBR=eseidel@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178561

[Mikhail, 2014-07-21 12:56:11]

Created Revert of Remove user-defined move constructor and assignment operator
from WTF::String

[commit-bot: I haz the power, 2014-07-21 12:56:50]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mikhail.pozdnyakov@intel.com/402163002/1

[commit-bot: I haz the power, 2014-07-21 12:56:50]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-07-21 12:56:51]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Inactive, 2014-07-21 12:59:45]

On 2014/07/21 12:56:51, I haz the power (commit-bot) wrote:
> No LGTM from a valid reviewer yet. Only full committers are accepted.
> Even if an LGTM may have been provided, it was from a non-committer or
> a provisional committer, _not_ a full super star committer.
> See http://www.chromium.org/getting-involved/become-a-committer
> Note that this has nothing to do with OWNERS files.

TBR=eseidel@chromium.org ?

[Mikhail, 2014-07-21 13:02:39]

The CQ bit was checked by mikhail.pozdnyakov@intel.com

[commit-bot: I haz the power, 2014-07-21 13:03:00]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mikhail.pozdnyakov@intel.com/402163002/1

[commit-bot: I haz the power, 2014-07-21 14:23:52]

Change committed as 178561

[Nico, 2014-07-21 14:31:49]

What was the problem with this commit?

[Inactive, 2014-07-21 14:34:34]

On 2014/07/21 14:31:49, Nico (away) wrote:
> What was the problem with this commit?

I explained on the original CL but the move constructor does not get generated
because we have a user defined destructor.

[Mikhail, 2014-07-21 14:37:23]

On 2014/07/21 14:31:49, Nico (away) wrote:
> What was the problem with this commit?

The problem is the WTF::String user-defined destructor which prevents from
generating of implicit move/copy ctors.
(Please see the original CL https://codereview.chromium.org/367343003/)

I've created a separate CL to remove the destructor
(https://codereview.chromium.org/402193002/) and if it sticks the reverted patch
can be applied again.