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-Commit-Position: refs/heads/master@{#19466}

webrtc/BUILD.gn

@@ -178,6 +178,7 @@ config("common_config") {
         "-Wthread-safety",
         "-Winconsistent-missing-override",
         "-Wundef",
+        "-Wunused-lambda-capture",
       ]
     }
   }