Reland "Roll buildtools and enable in-tree libc++ on Android"

This is a reland of 1a874fe60ccb87401867a7334b8eb11e5e4b1552
Android sanitizer bots should be green after:
https://chromium-review.googlesource.com/c/chromium/src/+/1387399

Original change's description:
> Roll buildtools and enable in-tree libc++ on Android
>
> Buildtools changelog:
> 7d88270de1..0e1cbc4eab
>
> BUG=767901
> R=thakis
>
> Change-Id: I96fdafca5e228cbfe7a0909170c0235e7845bbde
> Reviewed-on: https://chromium-review.googlesource.com/c/1385021
> Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#618035}

Bug: 767901
Change-Id: Ibcf1ee670f2696aeadb1a221022d21ddc4ca6753
Reviewed-on: https://chromium-review.googlesource.com/c/1387745
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#618427}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 10e1da1f3a8a3a03f3ff9c2a8939a497aae1af89

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"))