Enable custom libcxx unconditionally on CrOS.

Most other platforms either unconditionally, or are moving towards unconditionally
building with custom libcxx.

This comment suggests that there was always the intention to move
unconditionally to libc++:
https://bugs.chromium.org/p/chromium/issues/detail?id=554841#c3

And this bug suggests that we're already paying most of the engineering
cost for making this config work:
https://bugs.chromium.org/p/chromium/issues/detail?id=996754

Change-Id: I030f65b5921c4f5572fafcaf975eff24e804f3ed
Bug: 1043560
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2010226
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733362}

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

@@ -12,9 +12,7 @@ declare_args() {
   # is not supported.
   use_custom_libcxx =
       is_fuchsia || is_android || is_mac || (is_ios && !use_xcode_clang) ||
-      (is_win && is_clang) ||
-      (is_linux &&
-       (!is_chromeos || default_toolchain != "//build/toolchain/cros:target"))
+      (is_win && is_clang) || is_linux
 
   # Use libc++ instead of stdlibc++ when using the host_cpu toolchain, even if
   # use_custom_libcxx is false. This is useful for cross-compiles where a custom