Use "= delete" for DISALLOW_IMPLICIT_CONSTRUCTORS.

Use "= delete" for DISALLOW_IMPLICIT_CONSTRUCTORS.

This will guarantee the error to be at compile time, and not rely on
the call visibility (private).

This is part of a three-sided CL:
1. https://codereview.chromium.org/1211403004
2. <this>
3. https://codereview.chromium.org/1222143003

BUG=447156
Committed: https://crrev.com/8cd0029ed083a2d9792c7cefdcb6381eb375357f
Cr-Commit-Position: refs/heads/master@{#337298}

[mlamouri (slow - plz ping), 2015-07-02 13:38:39]

mlamouri@chromium.org changed reviewers:
+ thakis@chromium.org

[Nico, 2015-07-02 15:00:30]

lgtm

[Nico, 2015-07-02 15:00:56]

Thanks for doing this!

[mlamouri (slow - plz ping), 2015-07-03 08:57:14]

The CQ bit was checked by mlamouri@chromium.org

[commit-bot: I haz the power, 2015-07-03 08:57:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1212403003/1

[commit-bot: I haz the power, 2015-07-03 10:27:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-03 10:28:58]

Patchset 1 (id:??) landed as
https://crrev.com/8cd0029ed083a2d9792c7cefdcb6381eb375357f
Cr-Commit-Position: refs/heads/master@{#337298}