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..3e67106a9ae08e47f40d58f8a8dc3454dd88acb5 100644
--- a/webrtc/build/common.gypi
+++ b/webrtc/build/common.gypi
@@ -121,6 +121,10 @@
     # enable schannel on windows.
     'use_legacy_ssl_defaults%': 0,
 
+    # Determines whether NEON code will be built. It might not be selected at
+    # run-time.
+    'build_with_neon%': 0,
+
     'conditions': [
       ['build_with_chromium==1', {
         # Exclude pulse audio on Chromium since its prerequisites don't require
@@ -262,6 +266,7 @@
           'WEBRTC_ARCH_ARM64',
           'WEBRTC_HAS_NEON',
         ],
+        'variables': {'build_with_neon%': 1,},
       }],
       ['target_arch=="arm"', {
         'defines': [
@@ -273,9 +278,11 @@
             'conditions': [
               ['arm_neon==1', {
                 'defines': ['WEBRTC_HAS_NEON',],
+                'variables': {'build_with_neon%': 1,},
               }],
               ['arm_neon==0 and arm_neon_optional==1', {
                 'defines': ['WEBRTC_DETECT_NEON',],
+                'variables': {'build_with_neon%': 1,},
               }],
             ],
           }],