Implement $Secure- cookie prefix

This CL implements the rule that cookies whose names start with
$Secure- can only be set if the Secure attribute is enabled. The
implementation is a runtime-enabled web platform feature for now.

Intent to Implement:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/IU5t6eLuS2Y/Uq-7Kat9BwAJ

BUG=541511
TBR=droger@chromium.org

Review URL: https://codereview.chromium.org/1393193005

Cr-Commit-Position: refs/heads/master@{#354676}
12 files changed