Reading webstore_widget's manifest file is failing.
I'm looking into what causes this.

Ah you were faster. I added some logging and got:

[8300:8300:1206/163631.970222:ERROR:component_loader.cc(173)] Failed to parse extension manifest.
[8300:8300:1206/163631.970504:ERROR:component_loader.cc(174)] @@ name: Chrome Web Store Gallery
[8300:8300:1206/163631.970552:ERROR:component_loader.cc(175)] @@ description: An app that shows a filtered list of extensions from Chrome Web Store. Extensions in the list can be installed directly from the app.

manifest_contents was empty.

Looks that the manifest contents aren't included in the resource file?

% grep 'App for showing Chrome Webstore Gallery widget' out_kevin/Release/resources.pak
%

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/27d65fa315b9eaa62db39ab9f2244e27d54377a9

commit 27d65fa315b9eaa62db39ab9f2244e27d54377a9
Author: fukino <fukino@chromium.org>
Date: Wed Dec 07 04:44:37 2016

Add chrome_apps_resources.pak to resources.pak.

webstore_widget extension, which is a part of chrome_app_resources.pak, was not
included in resources.pak. It resulted in "Failed to parse extension manifest."
error.
chrome_app_resources.pak was added in
https://codereview.chromium.org/1103943003/diff/140001/chrome/chrome_repack_resources.gypi,
but it was included in resources.pak only in GYP build.
Doing the same for GN build fixes the issue.

BUG= 671521 
TEST=boot Chrome OS and confirm that "Failed to parse extension manifest.' is not observed.

Review-Url: https://codereview.chromium.org/2556443003
Cr-Commit-Position: refs/heads/master@{#436862}

[modify] https://crrev.com/27d65fa315b9eaa62db39ab9f2244e27d54377a9/chrome/chrome_paks.gni

 Issue 671514  has been merged into this issue.