Update webrtc/test to new VideoFrameBuffer interface

This is a follow-up cleanup for CL
https://codereview.webrtc.org/2847383002/.

TBR=stefan@webrtc.org

Bug: webrtc:7632
Change-Id: I8275e8edbd22b557cdb251f342847f4e8306299c
Reviewed-on: https://chromium-review.googlesource.com/524084
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18528}
diff --git a/webrtc/test/frame_generator_unittest.cc b/webrtc/test/frame_generator_unittest.cc
index 8867c01..1ead698 100644
--- a/webrtc/test/frame_generator_unittest.cc
+++ b/webrtc/test/frame_generator_unittest.cc
@@ -59,14 +59,16 @@
   void CheckFrameAndMutate(VideoFrame* frame, uint8_t y, uint8_t u, uint8_t v) {
     // Check that frame is valid, has the correct color and timestamp are clean.
     ASSERT_NE(nullptr, frame);
+    rtc::scoped_refptr<I420BufferInterface> i420_buffer =
+        frame->video_frame_buffer()->ToI420();
     const uint8_t* buffer;
-    buffer = frame->video_frame_buffer()->DataY();
+    buffer = i420_buffer->DataY();
     for (int i = 0; i < y_size; ++i)
       ASSERT_EQ(y, buffer[i]);
-    buffer = frame->video_frame_buffer()->DataU();
+    buffer = i420_buffer->DataU();
     for (int i = 0; i < uv_size; ++i)
       ASSERT_EQ(u, buffer[i]);
-    buffer = frame->video_frame_buffer()->DataV();
+    buffer = i420_buffer->DataV();
     for (int i = 0; i < uv_size; ++i)
       ASSERT_EQ(v, buffer[i]);
     EXPECT_EQ(0, frame->ntp_time_ms());