reland: Rename buildflag_headers for consistency

This renames targets in chrome, chromecast and chromeos.

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

BUG=814485
TEST=covered by build
TBR=thakis@chromium.org,dcheng@chromium.org

Change-Id: I35f82bb244d2f8bd745d3157f266bae99f21f888
Reviewed-on: https://chromium-review.googlesource.com/935296
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#538905}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6200d332518cd781f87ab86554bc71ff7a01ec4e

linux/unbundle/ffmpeg.gn

@@ -14,8 +14,8 @@ pkg_config("system_ffmpeg") {
   ]
 }
 
-buildflag_header("ffmpeg_features") {
-  header = "ffmpeg_features.h"
+buildflag_header("ffmpeg_buildflags") {
+  header = "ffmpeg_buildflags.h"
   flags = [ "USE_SYSTEM_FFMPEG=true" ]
 }
 
@@ -30,7 +30,7 @@ shim_headers("ffmpeg_shim") {
 
 source_set("ffmpeg") {
   deps = [
-    ":ffmpeg_features",
+    ":ffmpeg_buildflags",
     ":ffmpeg_shim",
   ]
   public_configs = [ ":system_ffmpeg" ]