Reenable libc++ on Windows

BUG=801780
R=thakis

Change-Id: Ife6f11c4e02534c879731274d427b0f563b8fa08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1556590
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652740}

build/config/c++/c++.gni

@@ -11,7 +11,7 @@ declare_args() {
   # Don't check in changes that set this to false for more platforms; doing so
   # is not supported.
   use_custom_libcxx =
-      is_fuchsia || is_android || is_mac || is_ios ||
+      is_fuchsia || is_android || is_mac || is_ios || (is_win && is_clang) ||
       (is_linux &&
        (!is_chromeos || default_toolchain != "//build/toolchain/cros:target"))