Rename buildflag_headers for consistency

This updates the remaining targets in ui/ and url/.

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

BUG=814485
TEST=covered by build

Change-Id: I746dee03434f9bf593ea19f4868f67fa2482f14b
Reviewed-on: https://chromium-review.googlesource.com/c/1405493
Commit-Queue: Scott Violet <sky@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621679}

diff --git a/ios/net/BUILD.gn b/ios/net/BUILD.gn
index 255e44c..bed74bbc 100644
--- a/ios/net/BUILD.gn
+++ b/ios/net/BUILD.gn

@@ -25,7 +25,7 @@
     ":ios_net_buildflags",
     "//base",
     "//net",
-    "//url:url_features",
+    "//url:buildflags",
   ]
 
   configs += [ "//build/config/compiler:enable_arc" ]