mac: Fix os_exchange_data_provider_mac.mm.

mac: Fix os_exchange_data_provider_mac.mm.

Provider::GetString is intended to return a plain-text representation of the
contents of the paste board. On OSX, it was possible for the paste board to
contain an NSURL, but not an NSString, and as a result Provider::GetString was
incorrectly returning the empty string.

This bug manifests itself when Chrome is compiled against the OSX 10.10 SDK. I
didn't trace down the exact source of the bug, although it is likely related to
https://codereview.chromium.org/1128173004.

BUG=463170
Committed: https://crrev.com/5c4eb6866ad14735e16e20b95f34fdea2863259c
Cr-Commit-Position: refs/heads/master@{#338405}

[erikchen, 2015-07-10 22:48:48]

erikchen@chromium.org changed reviewers:
+ avi@chromium.org

[erikchen, 2015-07-10 22:48:49]

avi: Please review.

[Avi (use Gerrit), 2015-07-10 23:13:17]

lgtm

[erikchen, 2015-07-10 23:13:45]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-07-10 23:14:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1235713004/1

[commit-bot: I haz the power, 2015-07-11 00:09:04]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-11 00:10:09]

Patchset 1 (id:??) landed as
https://crrev.com/5c4eb6866ad14735e16e20b95f34fdea2863259c
Cr-Commit-Position: refs/heads/master@{#338405}