Add a [rtc_]build_with_neon variable to unify conditions.

webrtc/build/webrtc.gni

Index: webrtc/build/webrtc.gni
diff --git a/webrtc/build/webrtc.gni b/webrtc/build/webrtc.gni
index d03a4ed8c98827df63f59c2ce465348e0ccd9831..fa059965dd4ee8020dc26d898859bf4593391544 100644
--- a/webrtc/build/webrtc.gni
+++ b/webrtc/build/webrtc.gni
@@ -105,10 +105,10 @@ declare_args() {
     rtc_use_openmax_dl = false
   }
 
-  # WebRTC builds ARM v7 Neon instruction set optimized code for both iOS and
-  # Android, which is why we currently cannot use the variables in
-  # //build/config/arm.gni (since it disables Neon for Android).
-  rtc_build_armv7_neon = current_cpu == "arm" && arm_version >= 7
+  # Determines whether NEON code will be built.
+  rtc_build_with_neon = (current_cpu == "arm" &&
+                         (arm_use_neon == 1 || arm_optionally_use_neon == 1)) ||
+                        current_cpu == "arm64"
 }
 
 # Make it possible to provide custom locations for some libraries (move these