Enable -Wunused-lambda-capture warning

This should prevent some build errors downstream.
The warning is currrently disabled in Chromium:
https://cs.chromium.org/chromium/src/build/config/compiler/BUILD.gn?rcl=735aa36217a394455f70aad87e2c844053fd810a&l=1214
(see https://crbug.com/681136 for more info).

BUG=webrtc:7133
NOTRY=True

Review-Url: https://codereview.webrtc.org/2999403002
Cr-Original-Commit-Position: refs/heads/master@{#19466}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: fd7cffa43366d5fb8a95da71fffa7f872037fefe
diff --git a/BUILD.gn b/BUILD.gn
index 1f7f37f..cb6c836 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -178,6 +178,7 @@
         "-Wthread-safety",
         "-Winconsistent-missing-override",
         "-Wundef",
+        "-Wunused-lambda-capture",
       ]
     }
   }