Turn on warnings for the extensions target.

Fix warnings that crept in since -Wall got dropped (in
https://codereview.chromium.org/23513035)

BUG=307668
R=yoz@chromium.org

Review URL: https://codereview.chromium.org/37203005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230500 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230500