GN: Fix iOS build.

This makes it possible to run GN for the iOS build, which is being
worked on for Chromium.

For the command below to work https://codereview.chromium.org/1314413006/
needs to be rolled into WebRTC.

TESTED=gn gen out/Default --args="build_with_chromium=false target_os=\"ios\""
BUG=chromium:459705
R=dpranke@chromium.org

Review URL: https://codereview.webrtc.org/1309663007 .

Cr-Commit-Position: refs/heads/master@{#9912}

webrtc/base/BUILD.gn

@@ -535,7 +535,7 @@ static_library("rtc_base") {
   }
 
   if (is_ios) {
-    all_dependent_configs += [ ":ios_config" ]
+    all_dependent_configs = [ ":ios_config" ]
 
     sources += [
       "macconversion.cc",

webrtc/modules/video_render/BUILD.gn

@@ -161,7 +161,7 @@ if (!build_with_chromium) {
 
       deps += [ "../..:webrtc_common" ]
 
-      cflags += [ "-fobjc-arc" ]  # CLANG_ENABLE_OBJC_ARC = YES.
+      cflags = [ "-fobjc-arc" ]  # CLANG_ENABLE_OBJC_ARC = YES.
     }
 
     all_dependent_configs = [ ":video_render_internal_impl_config" ]