Fix linker error due to missing MESSAGE_CENTER_EXPORT

Fix linker error due to missing MESSAGE_CENTER_EXPORT

Not sure in which condition this can be triggered, my environment is linux x64 with plain "component=shared_library clang=1", the actual linker invoked is "third_party/binutils/Linux_x64/Release/bin/ld.gold"

Here is the error log from linker:
/usr/lib64/gcc/x86_64-unknown-linux-gnu/4.9.1/../../../../include/c++/4.9.1/bits/stl_tree.h:1543: error: undefined reference to 'message_center::CompareTimestampSerial::operator()(message_center::Notification*, message_center::Notification*)'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

BUG=None

Committed: https://crrev.com/ac59b2cc31ddb0bf15074e6253fbf73196a39d69
Cr-Commit-Position: refs/heads/master@{#299933}

[binjin, 2014-10-15 01:41:25]

binjin@chromium.org changed reviewers:
+ stevenjb@chromium.org

[binjin, 2014-10-15 01:41:26]

Hello stevenjb,

Could please take a look at this CL?

-bjin

[stevenjb (google-dont-use), 2014-10-16 17:56:31]

stevenjb@google.com changed reviewers:
+ stevenjb@google.com

[stevenjb (google-dont-use), 2014-10-16 17:56:31]

lgtm

[stevenjb (google-dont-use), 2014-10-16 17:56:35]

The CQ bit was checked by stevenjb@google.com

[commit-bot: I haz the power, 2014-10-16 17:58:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/654093003/1

[binjin, 2014-10-16 17:59:57]

On 2014/10/16 17:56:31, stevenjb (google-dont-use) wrote:
> lgtm

Sorry for bothering, but could you use your chromium account? Since an owner
lgtm is required.

[commit-bot: I haz the power, 2014-10-16 18:07:03]

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

[commit-bot: I haz the power, 2014-10-16 18:07:04]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[stevenjb, 2014-10-16 18:12:17]

stevenjb@chromium.org changed reviewers:
- stevenjb@google.com

[stevenjb, 2014-10-16 18:12:18]

Ugh, sorry. lgtm.

[binjin, 2014-10-16 18:15:32]

The CQ bit was checked by binjin@chromium.org

[commit-bot: I haz the power, 2014-10-16 18:16:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/654093003/1

[commit-bot: I haz the power, 2014-10-16 18:21:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-16 18:22:35]

Patchset 1 (id:??) landed as
https://crrev.com/ac59b2cc31ddb0bf15074e6253fbf73196a39d69
Cr-Commit-Position: refs/heads/master@{#299933}