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
1 file changed