Add unittest for VideoFrameProviderClientImpl.

Class was woefully lacking in tests, coverage added! \o/

BUG=none
TEST=new unittest!
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1161513009

Cr-Commit-Position: refs/heads/master@{#333539}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 22df7d0..4f0a5ca5 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -791,6 +791,7 @@
     "layers/tiled_layer_unittest.cc",
     "layers/ui_resource_layer_impl_unittest.cc",
     "layers/ui_resource_layer_unittest.cc",
+    "layers/video_frame_provider_client_impl_unittest.cc",
     "layers/video_layer_impl_unittest.cc",
     "output/begin_frame_args_unittest.cc",
     "output/delegating_renderer_unittest.cc",