unbundle/ffmpeg: revert rename to ffmpeg_buildflags

ffmpeg_features is the target used in src/media/ffmpeg and elsewhere.

The target appears to have been accidentally renamed in:

    https://chromium-review.googlesource.com/935296

Fixes the following dependency errors when configured with system ffmpeg:

    ERROR Unresolved dependencies.
    //chrome/services/media_gallery_util:lib
      needs //third_party/ffmpeg:ffmpeg_features
    //chrome/services/media_gallery_util/public/cpp:browser_tests
      needs //third_party/ffmpeg:ffmpeg_features
    //chrome/services/media_gallery_util/public/cpp:cpp
      needs //third_party/ffmpeg:ffmpeg_features
    //media/base:base
      needs //third_party/ffmpeg:ffmpeg_features
    //media/ffmpeg:ffmpeg
      needs //third_party/ffmpeg:ffmpeg_features
    //media/filters:filters
      needs //third_party/ffmpeg:ffmpeg_features

Change-Id: I2218c1dfb20de87958eeb8c23b18eebbea384c36
Reviewed-on: https://chromium-review.googlesource.com/1131941
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573952}

build/linux/unbundle/ffmpeg.gn

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