Adding third_party/openh264/src, build files and unittests

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index 83d36b3a410658e2cd3b823a95cc47cdf7955805..e33af037ec47f056be95eb88eeadb49af4f32cbf 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -747,6 +747,11 @@ test("content_unittests") {
   if (is_mac && use_openssl) {
     deps += [ "//third_party/boringssl" ]
   }
+
+  if (build_openh264) {
+    sources += [ "../browser/media/openh264_unittests.cc" ]
+    deps += [ "//third_party/openh264/testing:openh264_testing" ]
+  }
 }
 
 test("content_perftests") {