Add usage message to event_log_visualizer.

Bug: webrtc:10616
Change-Id: I1623175abc7e60c1a3cf29e3d99fdde0e4140b1f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/146208
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28663}
diff --git a/rtc_tools/BUILD.gn b/rtc_tools/BUILD.gn
index 6f760654..c679c9e 100644
--- a/rtc_tools/BUILD.gn
+++ b/rtc_tools/BUILD.gn
@@ -337,8 +337,11 @@
         "../test:fileutils",
         "../test:test_support",
         "//third_party/abseil-cpp/absl/algorithm:container",
+        "//third_party/abseil-cpp/absl/flags:config",
         "//third_party/abseil-cpp/absl/flags:flag",
         "//third_party/abseil-cpp/absl/flags:parse",
+        "//third_party/abseil-cpp/absl/flags:usage",
+        "//third_party/abseil-cpp/absl/strings",
       ]
     }
   }