GN: Add -Wimplicit-fallthrough warning
GN: Add -Wimplicit-fallthrough warning

This warning was added for the GYP build back in
https://webrtc-codereview.appspot.com/34899004

NOTRY=True

Review URL: https://codereview.webrtc.org/1928543002

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#12531}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3f032cf6882b3438adab1940fe917c39a50fabbd

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc