Support widevine linux with bundle w/o default shipping

[simonhong]

To build with bundling, we should provide widevine lib and header.
gn checkes that two files during the generation.
Header file is used by brave-core to get latest version and
download url. However, lib file isn't used by brave-core.
So, empty fake lib file is added just for gn.
Real cdm lib is downloaded and initialized when user accepts the
use of widevine.

This pr should be merged with brave/brave-core#1606.

Fix #413

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Requested a security/privacy review as needed.

Test Plan:

1. Start browser with clean profile
2. Load https://bitmovin.com/demos/drm
3. Check No drm
4. Install widevine via content settings bubble
5. Check blocked image text is changed from not installed to not enabled and bubbles header
   and link text are also changed to not enabled and restart browser
6. Click restart browser link in bubble
7. Check browser is restarted and widevine is enabled

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions.