Move one last caller off ZipString.

(I haven't switched this to std::string because the code's already been
removed internally, so there's no point.)

Bug: http://b/129068177
Test: treehugger
Change-Id: I5675d65b2f74f06e68a9de570741a88ed1899eca
Merged-In: I723570315285d7c2caadffd71cd2d15efde11819