Creating webrtc:video_stream_api

This target keeps track of .h the files under webrtc/ that are not part
of any target.
If a .h file is not part of a target the 'gn check' utility is not
able to spot if a target is missing a dependency because even if
it parses '#include' directives it is not able to find a target that
contains these headers.

BUG=webrtc:7512
NOTRY=True

Review-Url: https://codereview.webrtc.org/2841873002
Cr-Commit-Position: refs/heads/master@{#17874}

webrtc/modules/video_coding/BUILD.gn

@@ -94,6 +94,7 @@ rtc_static_library("video_coding") {
     ":webrtc_i420",
     ":webrtc_vp8",
     ":webrtc_vp9",
+    "../..:video_stream_api",
     "../..:webrtc_common",
     "../../base:rtc_base",
     "../../base:rtc_base_approved",