[zip] Add default args to ZipReader's methods

Added sensible default arguments to:
ZipReader::ExtractCurrentEntryToString()
ZipReader::ExtractCurrentEntry()

Updated tests.

BUG=chromium:1295127
TEST=autoninja -C out/Default zlib_unittests && out/Default/zlib_unittests
TEST=autoninja -C out/Default components_unittests && out/Default/components_unittests --gtest_filter='UnzipTest.*'

Change-Id: Iec8d047e5eb777774b1aaef1f3c66f38db72a593
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3469122


Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/main@{#973621}
NOKEYCHECK=True
GitOrigin-RevId: bf5d8b34f48462995b919eb5428ef1b050ed0a00