Fixes
..\..\third_party\webrtc/base/stringutils.h(295,49) : warning(clang): extra qualification on member "empty_str" [-Wmicrosoft]
No behavior change, but makes the code more standards-conformant.
BUG=chromium:505296
Review URL: https://codereview.webrtc.org/1228193002
Cr-Commit-Position: refs/heads/master@{#9562}