[unpacker] Allow only one ZIP handler at once: either the new or the old one.

[unpacker] Allow only one ZIP handler at once: either the new or the old one.

If the new ZIP unpacker flow is enabled, then it the previous avfs based
logic should be hidden from user. And the other way the same.

TEST=Tested manually with the flag enabled and disabled.
BUG=427778
Committed: https://crrev.com/a5776f154ac000c7ce301227ecf5ccf748ab1650
Cr-Commit-Position: refs/heads/master@{#302265}

[mtomasz, 2014-10-30 08:02:49]

mtomasz@chromium.org changed reviewers:
+ fukino@chromium.org

[mtomasz, 2014-10-30 08:02:58]

@fukino: PTAL. Thanks!

[fukino, 2014-10-31 13:57:46]

On 2014/10/30 08:02:58, mtomasz wrote:
> @fukino: PTAL. Thanks!

lgtm.

[mtomasz, 2014-10-31 13:58:22]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2014-10-31 13:58:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/692793002/1

[commit-bot: I haz the power, 2014-10-31 14:41:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-31 14:41:55]

Patchset 1 (id:??) landed as
https://crrev.com/a5776f154ac000c7ce301227ecf5ccf748ab1650
Cr-Commit-Position: refs/heads/master@{#302265}