Followup for bug when zipping a folder with non-ascii chars in its name.

The commit adds a unit test and replaces the substr with the more
stylistically adequate FilePath::AppendRelativePath.

BUG=248115
TEST=ZipTest.ZipNonASCIIDir

Review URL: https://codereview.chromium.org/106663006

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ad4fc2d7d4b6c6415aaa2ef422c8f896544a02fa