New DEPS entry for //components/unexportable_keys

[Daniel Rubery]

Hello,

I'd like to add a new entry in //content/browser/DEPS for //components/unexportable_keys. This will be done in https://crrev.com/c/6198936, as part of the integration of BFCache and the new web platform feature Device Bound Session Credentials. I think this is a reasonable inclusion as DBSC is a web platform launch and therefore the new component is "related to the implementation of the web platform" (quoting from here).

Technically the dependency is only used for testing in that CL, so I am open to some alternative solutions (for example we could create new BFCache tests in //chrome). But given there is already an implicit dependency between //content and //components/unexportable_keys, I'd prefer the simple solution of declaring the DEPS.

Thanks,

Dan Rubery