Looking at this, there seems to be a bunch of options:
- ExtensionErrorReporter can display load time errors, but not warnings.
- Extension::AddInstallWarning can display warnings, but the warnings are associated with the manifest.
- ErrorConsole can add manifest (e.g. bad format for manifest keys), runtime (goes to the error console) or internal errors (LOG messages).

Is there an existing way of adding warnings? Extension::AddInstallWarning seems to be the closest to me.

AddInstallWarning is the right thing to do for this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a98b44daed1683b35f0b5b638f5693aebf0dab5a

commit a98b44daed1683b35f0b5b638f5693aebf0dab5a
Author: meacer <meacer@chromium.org>
Date: Tue Nov 22 21:25:55 2016

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-Commit-Position: refs/heads/master@{#433984}

[modify] https://crrev.com/a98b44daed1683b35f0b5b638f5693aebf0dab5a/extensions/utility/utility_handler.cc
[modify] https://crrev.com/a98b44daed1683b35f0b5b638f5693aebf0dab5a/third_party/zlib/google/zip.cc
[modify] https://crrev.com/a98b44daed1683b35f0b5b638f5693aebf0dab5a/third_party/zlib/google/zip.h

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

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For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot