Reland "Move definition of kWebRtcH264WithOpenH264FFmpeg to Blink"

This is a reland of f8ab5df96b5ddc2f17b8f7bc37337f81d80bbe9c

The original (reverted CL) https://crrev.com/c/1778671 had one mistake:
it missed to remove the includes of the autogenerated file

  <out>/gen/content/public/common/buildflags.h in chrome/.

... that is now gone. Given that the bots do not do a full rebuild
(remove <out>/), CQ did not catch it.
This reland CL should fix it.

Original change's description:
> Move definition of kWebRtcH264WithOpenH264FFmpeg to Blink
>
> This is a precursor CL to Onion soup'ing
> content::RtcVideoEncoderFactory (phase 2.3 in the design document [1]).
>
> [1] https://docs.google.com/document/d/1AJKVA5U4nDkyDB9p4ROrggWXadCxyy-grKaE9KS5vOU/
>
> Basically, it merged the definition and declaration of
> content/public/common/feature_h264_with_openh264_ffmpeg.cc|h
> into Blink's features.h. More specifically, to
> third_party/blink/public/common/features.h (declaration) and
> third_party/blink/common/features.cc (definition).
>
> This allows rtc_video_encoder_factory.cc|h to move to Blink in a
> follow up step.
>
> BUG=787254
> [email protected], [email protected], [email protected]
>
> Change-Id: I4ba7c59da0f799e1d30053923fb1e692202223b6
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1778671
> Commit-Queue: Antonio Gomes <[email protected]>
> Reviewed-by: Kentaro Hara <[email protected]>
> Reviewed-by: Guido Urdaneta <[email protected]>
> Reviewed-by: John Abd-El-Malek <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#692757}

[email protected]

Bug: 787254
Change-Id: I479b07d13802e3cad25b57e8c7dbeee048db0fd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1782514
Commit-Queue: Antonio Gomes <[email protected]>
Reviewed-by: François Doray <[email protected]>
Cr-Commit-Position: refs/heads/master@{#692861}
diff --git a/third_party/blink/common/features.cc b/third_party/blink/common/features.cc
index b1590bd..2bf2784 100644
--- a/third_party/blink/common/features.cc
+++ b/third_party/blink/common/features.cc
@@ -179,6 +179,12 @@
 const base::Feature kRTCOfferExtmapAllowMixed{
     "RTCOfferExtmapAllowMixed", base::FEATURE_DISABLED_BY_DEFAULT};
 
+#if BUILDFLAG(RTC_USE_H264) && BUILDFLAG(ENABLE_FFMPEG_VIDEO_DECODERS)
+// Run-time feature for the |rtc_use_h264| encoder/decoder.
+const base::Feature kWebRtcH264WithOpenH264FFmpeg{
+    "WebRTC-H264WithOpenH264FFmpeg", base::FEATURE_ENABLED_BY_DEFAULT};
+#endif  // BUILDFLAG(RTC_USE_H264) && BUILDFLAG(ENABLE_FFMPEG_VIDEO_DECODERS)
+
 const base::Feature kServiceWorkerIsolateInForeground{
     "ServiceWorkerIsolateInForeground", base::FEATURE_ENABLED_BY_DEFAULT};