Credential Manager: Add platform-exposed Credential objects.

We're going to need to pass credentials up into Chromium, and back down
into Blink. This patch makes that possible by adding a shared
"Platform*Credential" type which can be used by both the Blink module's
"*Credential" objects, and the platform's "Platform*Credential" objects.

BUG=400674

Review URL: https://codereview.chromium.org/457683002

git-svn-id: svn://svn.chromium.org/blink/trunk@179919 bbb929c8-8fbe-4397-9dbb-9b2b20218538