libvpx.gyp: try again to fix Clang's -Wstring-conversion warning

Using -Wno-conversion didn't work, since -Wstring-conversion was added
afterwards. Using clang_warning_flags_unset should work better.

BUG=431803

Review URL: https://codereview.chromium.org/786543002

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/libvpx@293280 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
1 file changed