New ZipReader::ExtractCurrentEntryToString API.

New API which extract a zip entry into a memory buffer.

BUG=359428

Review URL: https://codereview.chromium.org/292443006

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: aeea2725ce362b96bb2a47d5b218e69e4aa34c3c