Set use_cxx to false.

Bug: chromium:1402249
Change-Id: Ibf2a12f8771c7d6d5f0735e2cb50d6a8963a0b9f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/305225
Commit-Queue: Andrey Logvin <landrey@webrtc.org>
Auto-Submit: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Andrey Logvin <landrey@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40067}

.gn

diff --git a/.gn b/.gn
index 445772f..884a7f7 100644
--- a/.gn
+++ b/.gn
@@ -75,4 +75,6 @@
   fuchsia_sdk_readelf_exec =
       "//third_party/llvm-build/Release+Asserts/bin/llvm-readelf"
   fuchsia_target_api_level = 9
+
+  use_cxx17 = false
 }