Move video_encoder.h and video_decoder.h to /api and create GN targets for them

webrtc/api/BUILD.gn

Index: webrtc/api/BUILD.gn
diff --git a/webrtc/api/BUILD.gn b/webrtc/api/BUILD.gn
index 71e5243f90336e3f614fd1bf42f739a52ac304d2..86711c0dd2fdf33a3603b5a847994376917d5234 100644
--- a/webrtc/api/BUILD.gn
+++ b/webrtc/api/BUILD.gn
@@ -34,6 +34,30 @@ rtc_source_set("call_api") {
   ]
 }
 
+rtc_source_set("video_decoder_api") {
+  sources = [
+    "video_decoder.h",
+  ]
+
+  deps = [
+    "..:webrtc_common",
+  ]
+}
+
+rtc_source_set("video_encoder_api") {
+  sources = [
+    "video_encoder.h",
+  ]
+
+  deps = [
+    "..:webrtc_common",
+    "../base:rtc_base_approved",
+
+    # TODO(ilnik): Add dependency on webrtc/video_frame.h when it will have it's
+    # own build target.
+  ]
+}
+
 rtc_static_library("libjingle_peerconnection_api") {
   check_includes = false  # TODO(kjellander): Remove (bugs.webrtc.org/6828)
   cflags = []