linux and chromeos: Turn on -Wunused-const-variable.

BUG=307668
R=hans@chromium.org
TBR=oshima

Review URL: https://codereview.chromium.org/29263007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229578 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229578