Use zlib library instead of gzip library.
Use zlib library instead of gzip library.

It turns out that the gzip python library is horribly slow.
Instead of using gzip to compress individual files,
use the zlib library with gzip compatible configuration.
This decreases the runtime of package_index.py from
about 90 minutes to about 3m40s.

BUG=429684

Review URL: https://codereview.chromium.org/881693004

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@293969 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build