GN: Implement BUILD.gn for common_video.

This adds copying of Chromium's third_party/BUILD.gn
to acommondate libyuv's BUILD.gn that imports the 'jpeg'
config from that file.

BUG=3441
TEST=trybots + local compile passing with:
gn gen out/Default --args="build_with_chromium=false" && ninja -C out/Default
gn gen out/Default --args="build_with_chromium=false build_libyuv=false" && ninja -C out/Default

R=niklas.enbom@webrtc.org, pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/14839004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6595 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/DEPS b/DEPS
index 485affa..8629afb 100644
--- a/DEPS
+++ b/DEPS
@@ -48,6 +48,9 @@
   "third_party/binutils":
     Var("chromium_trunk") + "/src/third_party/binutils@" + Var("chromium_revision"),
 
+  "third_party/build_gn":
+    File(Var("chromium_trunk") + "/src/third_party/BUILD.gn@" + Var("chromium_revision")),
+
   "third_party/clang_format":
     Var("chromium_trunk") + "/src/third_party/clang_format@" + Var("chromium_revision"),
 
@@ -244,6 +247,14 @@
                Var("root_dir") + "/../chromium_gn/.gn",
                Var("root_dir")],
   },
+  {
+    # Copy BUILD.gn from temporary place (third_party/build_gn) to third_party.
+    "name": "copy third_party/BUILD.gn",
+    "pattern": ".",
+    "action": ["python", Var("root_dir") + "/build/cp.py",
+               Var("root_dir") + "/third_party/build_gn/BUILD.gn",
+               Var("root_dir") + "/third_party"],
+  },
   # Pull GN binaries. This needs to be before running GYP below.
   {
     "name": "gn_win",