Rename buildflag_headers for consistency

This updates ios.

Targets go from foo_features to foo_buildflags
And similarly the header now has buildflags in the name.

BUG=814485
TEST=covered by build

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I7a59eb0e91b3813855e79352a6146f4d1410c85a
Reviewed-on: https://chromium-review.googlesource.com/981322
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545932}

diff --git a/ios/net/BUILD.gn b/ios/net/BUILD.gn
index 28b8845..8b514af 100644
--- a/ios/net/BUILD.gn
+++ b/ios/net/BUILD.gn

@@ -8,8 +8,8 @@
 import("//testing/test.gni")
 import("//url/features.gni")
 
-buildflag_header("ios_net_features") {
-  header = "ios_net_features.h"
+buildflag_header("ios_net_buildflags") {
+  header = "ios_net_buildflags.h"
   flags = [ "CRONET_BUILD=$is_cronet_build" ]
 }
 
@@ -22,7 +22,7 @@
 
 source_set("net") {
   deps = [
-    ":ios_net_features",
+    ":ios_net_buildflags",
     "//base",
     "//net",
     "//url:url_features",