Switch event_log_visualizer to ABSL_FLAG.

Bug: webrtc:10616
Change-Id: I915ce581b082e081501f4e8545e939a81baff366
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/143968
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28575}
diff --git a/rtc_tools/BUILD.gn b/rtc_tools/BUILD.gn
index 63a8b69..f684528 100644
--- a/rtc_tools/BUILD.gn
+++ b/rtc_tools/BUILD.gn
@@ -358,6 +358,8 @@
         "../test:fileutils",
         "../test:test_support",
         "//third_party/abseil-cpp/absl/algorithm:container",
+        "//third_party/abseil-cpp/absl/flags:flag",
+        "//third_party/abseil-cpp/absl/flags:parse",
       ]
     }
   }