Fix TopSitesObserver implementation

Fix TopSitesObserver implementation

Add a virtual destructor since all class with a virtual method should
have a virtual destructor.

Disable copy constructor and assignment operator using the macro
DISALLOW_COPY_AND_ASSIGN since you don't want to do slicing when
using a class with virtual methods.

BUG=None

Committed: https://crrev.com/44640d47d57bb9a219fc0a469d9e443225d30445
Cr-Commit-Position: refs/heads/master@{#311473}

[sdefresne, 2015-01-14 12:19:55]

sdefresne@chromium.org changed reviewers:
+ droger@chromium.org

[sdefresne, 2015-01-14 12:19:56]

Please take a look.

[droger, 2015-01-14 12:28:58]

lgtm

[sdefresne, 2015-01-14 12:49:08]

The CQ bit was checked by sdefresne@chromium.org

[commit-bot: I haz the power, 2015-01-14 12:49:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/853643003/1

[commit-bot: I haz the power, 2015-01-14 12:53:55]

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

[commit-bot: I haz the power, 2015-01-14 12:53:55]

Try jobs failed on following builders:
  ios_dbg_simulator on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[sdefresne, 2015-01-14 14:07:25]

The CQ bit was checked by sdefresne@chromium.org

[commit-bot: I haz the power, 2015-01-14 14:08:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/853643003/1

[commit-bot: I haz the power, 2015-01-14 14:38:09]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-14 14:39:14]

Patchset 1 (id:??) landed as
https://crrev.com/44640d47d57bb9a219fc0a469d9e443225d30445
Cr-Commit-Position: refs/heads/master@{#311473}