Override C++ standard to -std=gnu++11

Chromium switched to C++14 but WebRTC does not want to follow suit.
This enables the override added in
https://chromium-review.googlesource.com/c/596870

BUG=chromium:750683
NOTRY=True

Review-Url: https://codereview.webrtc.org/2992823002
Cr-Commit-Position: refs/heads/master@{#19207}
diff --git a/.gn b/.gn
index 0079aaa..0a15148 100644
--- a/.gn
+++ b/.gn
@@ -38,4 +38,7 @@
   is_component_build = false
 
   mac_sdk_min = "10.11"
+
+  # WebRTC does not want to switch to C++14 yet.
+  use_cxx11 = true
 }