Android: Enable in-tree libc++ builds

BUG=767901
R=thakis
CC=pcc

Change-Id: Ib394126f8e729192ee41520eefd85e22531bfd65
Reviewed-on: https://chromium-review.googlesource.com/1066596
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561361}

build/config/android/BUILD.gn

@@ -115,7 +115,6 @@ config("runtime_library") {
   # caution.
   cflags_cc = []
   if (!use_custom_libcxx) {
-    cflags_cc = []
     if (android_ndk_major_version >= 13) {
       libcxx_include_path =
           rebase_path("$android_libcpp_root/include", root_build_dir)

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_msan || is_fuchsia ||
+      is_msan || is_fuchsia || is_android ||
       (is_linux &&
        (!is_chromeos || default_toolchain != "//build/toolchain/cros:target"))