Add k prefix to FrameGenerator::OutputType enum values

This prepares for using VideoFrameBuffer::Type as
FrameGenerator::OutputType, which will reduce the
number of redundant enums in the code.

Bug: webrtc:9883
Change-Id: I253f5f1ea7181e02a5cf1a92925f51da8ada6aa2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/146982
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28696}

modules/video_coding/codecs/test/video_codec_unittest.cc

@@ -75,7 +75,7 @@ void VideoCodecUnitTest::SetUp() {
 
   input_frame_generator_ = test::FrameGenerator::CreateSquareGenerator(
       codec_settings_.width, codec_settings_.height,
-      test::FrameGenerator::OutputType::I420, absl::optional<int>());
+      test::FrameGenerator::OutputType::kI420, absl::optional<int>());
 
   encoder_ = CreateEncoder();
   decoder_ = CreateDecoder();