Silence the overly verbose skipped files warning when extracting CRX files

CRX files are extracted twice: Once for only manifest.json, once for the rest
of the contents. Extracting manifest.json causes a lot of noise in debug builds
since all other files are skipped. Suppress the log statements when extracting
manifest.json.

BUG=645263

Review-Url: https://codereview.chromium.org/2523803002
Cr-Original-Commit-Position: refs/heads/master@{#433984}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a98b44daed1683b35f0b5b638f5693aebf0dab5a