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When refactoring code it is easy to accidentally disregard the output of
base::StringPrintf. The code will still look superficially correct, and may even
pass code review (as happened with https://codereview.chromium.org/436133002/ ).

This type of bug can be prevented by annotating StringPrintf with
WARN_UNUSED_RESULT. Despite the name, this causes a compile error in our build
configuration (except on Windows, where it is ignored). This should ensure that
such errors are caught by the CQ if not sooner.

TEST=build all; base_unittests
BUG=400663

Review URL: https://codereview.chromium.org/440013003

Cr-Commit-Position: refs/heads/master@{#288353}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288353 0039d316-1c4b-4281-b951-d872f2087c98