Mojo Mutex: fix declspec and GNU attribute order

Clang can't parse this:

   class __declspec(dllexport) __attribute__((lockable)) Mutex { };

the attribute and declspec have to come in the other order.

BUG=82385
R=rockot@chromium.org

Review URL: https://codereview.chromium.org/1310423002 .

Cr-Commit-Position: refs/heads/master@{#345202}
1 file changed