DocumentThreadableLoader: Rename isAllowedByPolicy() to isAllowedByContentSecurityPolicy()

We have many .*Policy in this file. Use a more specific name for
readability.

R=mkwst
BUG=none

Review URL: https://codereview.chromium.org/666533005

git-svn-id: svn://svn.chromium.org/blink/trunk@184352 bbb929c8-8fbe-4397-9dbb-9b2b20218538