UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2661.86 Safari/537.36

Steps to reproduce the problem:
1. Install google chrome on a debian 64bits system
2. Run `apt update`

I believe that there is a problem with google chrome debian repository. For a while it shows a warning every time I run the update in apt (`apt update`). But the last time I updated (few hours ago), it shows a error of a missing key, from the Google Chrome repo.

What is the expected behavior?
The `apt update` finishes without fail 

What went wrong?
In the end of the `apt update` command there is a error of a unknown key

```
(...)
# apt update
Fetched 3,440 B in 8s (396 B/s)                                                                                           
Reading package lists... Done
Building dependency tree       
Reading state information... Done
83 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: http://dl.google.com/linux/chrome/deb/dists/stable/Release.gpg: Signature by key 4CCA1EAF950CEE4AB83976DCA040830F7FAC5991 uses weak digest algorithm (SHA1)
W: There is no public key available for the following key IDs:
1397BC53640DB551
```

Did this work before? N/A 

Chrome version: 50.0.2661.86  Channel: stable
OS Version: Debian Sid (Unstable)
Flash Version: Shockwave Flash 21.0 r0

My apt config file

```
# cat google-chrome.list 
### THIS FILE IS AUTOMATICALLY CONFIGURED ###
# You may comment out this entry, but any other modifications may be lost.
deb [arch=amd64] http://dl.google.com/linux/chrome/deb/ stable main
```

Info about the package installed here.
```
# apt show google-chrome-stable
Package: google-chrome-stable
Version: 50.0.2661.86-1
Priority: optional
Section: web
Maintainer: Chrome Linux Team <chromium-dev@chromium.org>
Installed-Size: 186 MB
Provides: www-browser
Pre-Depends: dpkg (>= 1.14.0)
Depends: gconf-service, libasound2 (>= 1.0.23), libatk1.0-0 (>= 1.12.4), libc6 (>= 2.12), libcairo2 (>= 1.6.0), libcups2 (>= 1.4.0), libdbus-1-3 (>= 1.2.14), libexpat1 (>= 1.95.8), libfontconfig1 (>= 2.8.0), libfreetype6 (>= 2.3.9), libgcc1 (>= 1:4.1.1), libgconf-2-4 (>= 2.31.1), libgdk-pixbuf2.0-0 (>= 2.22.0), libglib2.0-0 (>= 2.26.0), libgtk2.0-0 (>= 2.24.0), libnspr4 (>= 1.8.0.10), libnss3 (>= 3.17.2), libpango1.0-0 (>= 1.14.0), libstdc++6 (>= 4.8.0), libx11-6 (>= 2:1.4.99.1), libxcomposite1 (>= 1:0.3-1), libxcursor1 (>> 1.1.2), libxdamage1 (>= 1:1.1), libxext6, libxfixes3, libxi6 (>= 2:1.2.99.4), libxrandr2 (>= 2:1.2.99.2), libxrender1, libxss1, libxtst6, ca-certificates, fonts-liberation, libappindicator1, libcurl3, lsb-base (>= 4.1), xdg-utils (>= 1.0.2), wget
Download-Size: 48.4 MB
APT-Manual-Installed: yes
APT-Sources: http://dl.google.com/linux/chrome/deb stable/main amd64 Packages
Description: The web browser from Google
 Google Chrome is a browser that combines a minimal design with sophisticated technology to make the web faster, safer, and easier.
```

With the google repository disabled (comment the apt source line), the `apt update` command runs ok.

Other people reported the same on Ubuntu and Linux Mint:
http://ubuntuforums.org/showthread.php?t=2322426&p=13479192&posted=1#post13479192
http://ubuntuforums.org/showthread.php?t=2322420&p=13479193&posted=1#post13479193
http://software.techforums.space/software/aptget-update-error-for-chrome-bccbdd44.html
https://www.reddit.com/r/linux4noobs/comments/4grdo7/an_error_occurred_w_there_is_no_public_key/

Those are all results I get now if I google for 1397BC53640DB551. The reddit link have some "solution", basically updating the keys manually from google, but this shouldn't happen nor need any user intervention.