Switch rgba_to_i420_converter to ABSL_FLAG.

Bug: webrtc:10616
Change-Id: I45f94e0f43e84b041c72b1bdc5b35ee0e970397e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/143179
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28362}
diff --git a/rtc_tools/BUILD.gn b/rtc_tools/BUILD.gn
index e3b9eb1..9df3843 100644
--- a/rtc_tools/BUILD.gn
+++ b/rtc_tools/BUILD.gn
@@ -242,6 +242,7 @@
 
   rtc_executable("rgba_to_i420_converter") {
     visibility = [ "*" ]
+    testonly = true
     sources = [
       "converter/converter.cc",
       "converter/converter.h",
@@ -249,8 +250,9 @@
     ]
 
     deps = [
-      ":command_line_parser",
       "../common_video",
+      "//third_party/abseil-cpp/absl/flags:flag",
+      "//third_party/abseil-cpp/absl/flags:parse",
       "//third_party/libyuv",
     ]
   }