Allow assertions to be enabled in Blink Release builds.

Source/build/features.gypi

Index: Source/build/features.gypi
diff --git a/Source/build/features.gypi b/Source/build/features.gypi
index d340677e7b98e44f4f661e1e1643738a7db60ce1..7eec2d47fe69e394239bdebe13178e69304c81d6 100644
--- a/Source/build/features.gypi
+++ b/Source/build/features.gypi
@@ -43,7 +43,8 @@
     'variables': {
       # Enables the Oilpan garbage-collection infrastructure.
       'enable_oilpan%': 0,
-      'gc_tracing%': 0
+      'gc_tracing%': 0,
+      'blink_asserts_always_on%': 0,
     },
     'conditions': [
       ['use_concatenated_impulse_responses==1', {
@@ -91,6 +92,11 @@
           'ENABLE_GC_TRACING=1',
         ],
       }],
+      ['blink_asserts_always_on==1', {
+        'feature_defines': [
+          'ENABLE_ASSERT=1',
+        ],
+      }],
     ],
   },
 }