icu: Warning tweaks in preparation for the next clang roll.

BUG=chromium:84851,chromium:102390
Review URL: https://chromiumcodereview.appspot.com/9417025

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/icu46@122379 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
1 file changed