LibvpxVp8Encoder: Clarify RTC_LOG error message.

While debugging https://crbug.com/1195144 I found it useful to clarify
this log statement.

The log would say "When scaling [kNative], the image was unexpectedly
converted to [kI420]..." but not saying what it was trying to convert
it to. This CL adds: "... instead of [kNV12]."

Bug: chromium:1195144
Change-Id: I13e0040edf5d7d98d80ce674812f67dfb73be36e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/214040
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33634}

modules/video_coding/codecs/vp8/libvpx_vp8_encoder.cc

@@ -1383,6 +1383,8 @@ LibvpxVp8Encoder::PrepareBuffers(rtc::scoped_refptr<VideoFrameBuffer> buffer) {
                         << VideoFrameBufferTypeToString(buffer_to_scale->type())
                         << ", the image was unexpectedly converted to "
                         << VideoFrameBufferTypeToString(scaled_buffer->type())
+                        << " instead of "
+                        << VideoFrameBufferTypeToString(mapped_buffer->type())
                         << ". Can't encode frame.";
       return {};
     }