Remove a superfluous qualifier on an inline method.

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}
1 file changed