This macro is used for defining static data members with
STATIC_CONST_MEMBER_DEFINITION. Clang-cl mimics MSVC's
behaviour here, so it also needs __declspec(selectany).

This change was prompted by Clang r237787 which changed
a bug where Clang would previously not emit symbols for
some static data members.

BUG=82385
LOG=N

Review URL: https://codereview.chromium.org/1145213004

Cr-Commit-Position: refs/heads/master@{#28563}