Fix //third_party/cec to not disable -Wunused-function.
Move the -Wno-unused-function to a private config that is not exported
via public_configs to avoid disabling the corresponding warning for all
targets depending on //base/test:test_support (//third_party/cec is a
public_deps of //base/test:test_support).
BUG=
666723
Committed:
https://crrev.com/bfd9c73b16d0bdb92ab04b1d5606fe82fe5fdac2
Cr-Commit-Position: refs/heads/master@{#434180}