Properly set size based on actual buffer layout

VP9FrameSizeTestsLarge.OneByOneVideo has been causing a failure in
jenkins libvpx__unit_tests-valgrind_long for "using of uninitialized
memory", the root cause was that the input image for this test was
not initialized with proper size, therefore plan U and V were not
initialized at all.

This commit fixes the size initialization, and resolves the issue.

Change-Id: Ic4dd1542b7bb0cb260a1e0aeeb505db21ae5edc8

test/encode_test_driver.cc

@@ -64,8 +64,7 @@ void Encoder::EncodeFrameInternal(const VideoSource &video,
 
   // Encode the frame
   API_REGISTER_STATE_CHECK(
-      res = vpx_codec_encode(&encoder_,
-                             video.img(), video.pts(), video.duration(),
+      res = vpx_codec_encode(&encoder_, img, video.pts(), video.duration(),
                              frame_flags, deadline_));
   ASSERT_EQ(VPX_CODEC_OK, res) << EncoderError();
 }