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}

build/config/compiler/BUILD.gn

@@ -13,6 +13,9 @@ if (current_cpu == "mipsel" || current_cpu == "mips64el") {
 if (is_posix) {
   import("//build/config/gcc/gcc_version.gni")
 }
+if (is_win) {
+  import("//build/config/win/visual_studio_version.gni")
+}
 
 import("//build/toolchain/ccache.gni")
 import("//build/config/sanitizers/sanitizers.gni")