Add ZipReader::ExtractCurrentEntry with a delegate interface.

This change gives consumers of ZipReader a way to have the current entry
streamed to them via a Delegate interface. It also:

- Reduces duplication in the ExtractCurrentEntry* functions.
- Uses the heap rather than the stack for intermediate buffers.
- Changes ExtractCurrentEntryToFd to ExtractCurrentEntryToFile, making
  it cross-platform in the process.

BUG=462584

Review URL: https://codereview.chromium.org/1014653002

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