Change QuicMemoryCacheBackend::InitializeDirectory() to handle cases where the specified directory contains only files, not subdirectories naming hosts. gfe-relnote: n/a - tools only https://bugs.chromium.org/p/chromium/issues/detail?id=1034470 PiperOrigin-RevId: 285875350 Change-Id: I352d71066f37d9a02280633ad439ad784c2531f2