Managed preferences for Chrome administrators

Implement support for managed preferences. This feature allows users
(administrators) to easily change the default settings of the PDF Viewer for
all Chrome or Chromium browsers within their organization.

External resources for end users (administrators)

- http://www.chromium.org/administrators/
- http://www.chromium.org/administrators/configuring-policy-for-extensions
- http://www.chromium.org/administrators/windows-quick-start
- http://www.chromium.org/administrators/mac-quick-start
- http://www.chromium.org/administrators/linux-quick-start
- http://www.chromium.org/administrators/policy-templates

Administrators can read one of the previous links to learn more about creating
policies. We want to auto-generate these templates, but there are no public
tools for doing that. It will be added in the future, see:
https://code.google.com/p/chromium/issues/detail?id=389061

Resources for PDF.js/extension developers

- http://cs.chromium.org/file:policy_templates.json
- https://developer.chrome.com/extensions/manifest/storage
Merge pull request #5082 from Rob--W/chromium-managed-preferences

Managed preferences for Chrome administrators