Try to fix the Win-Clang gn build

Builds were failing with:

  ERROR at //build/config/compiler/BUILD.gn:116:11: Undefined identifier
        if (visual_studio_version == "2013") {
            ^--------------------

BUG=82385, 440500

Review URL: https://codereview.chromium.org/1178603002

Cr-Commit-Position: refs/heads/master@{#333787}
1 file changed