Update CM API IDL to use SecureContext

This change implements recent changes to the Credential Manager API
specification that introduce the |SecureContext| extended attribute on
its interfaces. Furthermore it removes
|RuntimeEnabled=CredentialManager| from the interfaces following the
discussion in http://crrev.com/2832813002.

Intent to implement and ship:
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/-E-O9LgsPfM/ExJ7hiY1BQAJ

BUG=715077
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2844453002
Cr-Commit-Position: refs/heads/master@{#468596}
26 files changed