Reland of Adding backward compatibility header (patchset #1 id:1 of https://codereview.webrtc.org/2908143003/ )

Reason for revert:
Take three of relanding this after all internal issues have been resolved.

Original issue's description:
> Revert of Adding backward compatibility header (patchset #1 id:1 of https://codereview.webrtc.org/2909923002/ )
>
> Reason for revert:
> Breaks downstream project.
>
> Original issue's description:
> > Adding backward compatibility header
> >
> > This header will be removed ad soon as downstream projects will be
> > updated.
> >
> > BUG=webrtc:4867
> > NOTRY=True
> > TBR=kjellander@webrtc.org
> >
> > Review-Url: https://codereview.webrtc.org/2909923002
> > Cr-Commit-Position: refs/heads/master@{#18306}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/4fa8a97cc3268de73ad021ff56aaa9bf1a566ed4
>
> TBR=kjellander@webrtc.org,magjed@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:4867
>
> Review-Url: https://codereview.webrtc.org/2908143003
> Cr-Commit-Position: refs/heads/master@{#18308}
> Committed: https://chromium.googlesource.com/external/webrtc/+/6684fdce93ff25a837117cd7a811a6d86cbe4b20

TBR=kjellander@webrtc.org,magjed@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:4867

Review-Url: https://codereview.webrtc.org/2912773003
Cr-Commit-Position: refs/heads/master@{#18320}
diff --git a/webrtc/sdk/BUILD.gn b/webrtc/sdk/BUILD.gn
index 9b41a1e..c165d60 100644
--- a/webrtc/sdk/BUILD.gn
+++ b/webrtc/sdk/BUILD.gn
@@ -479,6 +479,9 @@
     sources = [
       "objc/Framework/Classes/Video/corevideo_frame_buffer.cc",
       "objc/Framework/Classes/Video/corevideo_frame_buffer.h",
+
+      # TODO(mbonadei): remove this backward compatibility header
+      "objc/Framework/Classes/corevideo_frame_buffer.h",
     ]
 
     deps = [
diff --git a/webrtc/sdk/objc/Framework/Classes/corevideo_frame_buffer.h b/webrtc/sdk/objc/Framework/Classes/corevideo_frame_buffer.h
new file mode 100644
index 0000000..bef9372
--- /dev/null
+++ b/webrtc/sdk/objc/Framework/Classes/corevideo_frame_buffer.h
@@ -0,0 +1,19 @@
+/*
+ *  Copyright 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_SDK_OBJC_FRAMEWORK_CLASSES_COREVIDEO_FRAME_BUFFER_H_
+#define WEBRTC_SDK_OBJC_FRAMEWORK_CLASSES_COREVIDEO_FRAME_BUFFER_H_
+
+// This is a backward compatibility header. It will be removed as soon as
+// dowstream projects will be updated.
+
+#include "webrtc/sdk/objc/Framework/Classes/Video/corevideo_frame_buffer.h"
+
+#endif  // WEBRTC_SDK_OBJC_FRAMEWORK_CLASSES_COREVIDEO_FRAME_BUFFER_H_