[GN] Create a non-null default native GL interface on iOS.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5212

Change-Id: Ie59cd79687af9eb2530fab8089477c1702e4badc
Reviewed-on: https://skia-review.googlesource.com/5212
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index a5b4d37..dca76eb 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -394,6 +394,8 @@
     sources += [ "src/gpu/gl/glx/GrGLCreateNativeInterface_glx.cpp" ]
   } else if (is_mac) {
     sources += [ "src/gpu/gl/mac/GrGLCreateNativeInterface_mac.cpp" ]
+  } else if (is_ios) {
+    sources += [ "src/gpu/gl/iOS/GrGLCreateNativeInterface_iOS.cpp" ]
   } else if (is_win) {
     sources += [ "src/gpu/gl/win/GrGLCreateNativeInterface_win.cpp" ]
     libs += [ "OpenGL32.lib" ]