Add a [rtc_]build_with_neon variable to unify conditions.

webrtc/build/common.gypi

Index: webrtc/build/common.gypi
diff --git a/webrtc/build/common.gypi b/webrtc/build/common.gypi
index 26b0de367a806f56f050ad386a50711e77c51b20..0ab88c63d4e029d7fad16fd387da4a8c4cc7b3c3 100644
--- a/webrtc/build/common.gypi
+++ b/webrtc/build/common.gypi
@@ -121,6 +121,9 @@
     # enable schannel on windows.
     'use_legacy_ssl_defaults%': 0,
 
+    # Determines whether NEON code will be built.
+    'build_with_neon%': 0,
+
     'conditions': [
       ['build_with_chromium==1', {
         # Exclude pulse audio on Chromium since its prerequisites don't require
@@ -152,6 +155,9 @@
       ['target_arch=="arm" or target_arch=="arm64"', {
         'prefer_fixed_point%': 1,
       }],
+      ['(target_arch=="arm" and (arm_neon==1 or arm_neon_optional==1)) or target_arch=="arm64"', {
+        'build_with_neon%': 1,
+      }],
       ['OS!="ios" and (target_arch!="arm" or arm_version>=7) and target_arch!="mips64el"', {
         'rtc_use_openmax_dl%': 1,
       }, {