(gcc) fix compilation of os_exchange_data_provider_aurax11.cc

This fixes a gcc compilation error in os_exchange_data_provider_aurax11.cc:
error: cannot convert 'ret' from type 'std::unique_ptr<ui::OSExchangeDataProviderAuraX11>'
    to type 'std::unique_ptr<ui::OSExchangeDataProviderAuraX11>&&'

Tested with gcc 4.8.4.

Followup to https://codereview.chromium.org/2179813003

BUG=614037

Review-Url: https://codereview.chromium.org/2192533002
Cr-Commit-Position: refs/heads/master@{#408233}
1 file changed