[WIP] support arm_neon_optional flag in blink.

Source/build/features.gypi

Index: Source/build/features.gypi
diff --git a/Source/build/features.gypi b/Source/build/features.gypi
index 5b23b5c76cf368943ff6c1c8f84284d29671160a..7df09b6b60e7a359b4cbbccb032aac2c854b13d1 100644
--- a/Source/build/features.gypi
+++ b/Source/build/features.gypi
@@ -47,6 +47,16 @@
       'blink_logging_always_on%': 0,
     },
     'conditions': [
+      ['arm_neon == 1', {
+        'feature_defines': [
+          'WTF_CPU_ARM_NEON',
+        ],
+      }],
+      ['arm_neon_optional == 1 and (OS=="linux" or OS=="android")', {
+        'feature_defines': [
+          'WTF_CPU_ARM_NEON_OPTIONAL',
+        ],
+      }],
       ['use_concatenated_impulse_responses==1', {
         # Use concatenated HRTF impulse responses
         'feature_defines': ['WTF_USE_CONCATENATED_IMPULSE_RESPONSES=1'],