GC plugin: have -Werror issue errors for inputs having only warnings.
If a translation unit contains only warnings, the GC plugin will issue
a warning diagnostic along with details/notes of the warnings encountered.
Have that warning be under the control of -Werror; the reason for not
having it as such up until now is that warnings have been far too plentiful
for the Blink codebase. This is no longer the case.
R=haraken,yutak,thakis
BUG=
Review URL: https://codereview.chromium.org/1481523005
Cr-Commit-Position: refs/heads/master@{#362128}
5 files changed
