Adding missing dependency on libyuv.

The build target //content/test:content_unittests was depending on
libyuv without declaring the dependecy in deps because the WebRTC
target //third_party/webrtc/api:video_frame_api was exposing it
via public_deps but in [1] public_deps will be replaced with deps.

[1] - https://webrtc-review.googlesource.com/c/src/+/28120

Bug: webrtc:8603
Change-Id: I3d0735e08f57b78a6f9d98c5ea1428f53f12a354
Reviewed-on: https://chromium-review.googlesource.com/805242Reviewed-by: Patrik Höglund <phoglund@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521319}

content/test/BUILD.gn

@@ -1836,6 +1836,7 @@ test("content_unittests") {
     ]
     deps += [
       "//services/device/public/interfaces",
+      "//third_party/libyuv",
       "//third_party/opus",
       "//third_party/webrtc/api:libjingle_peerconnection",
       "//third_party/webrtc/api:rtc_stats_api",