Rename allowCookies to allowStoredCredentials

And remove an redundant flag from WebURLRequest.
These two represent the same thing thus should be merged.

TEST=none
BUG=none
R=japhet@chromium.org, abarth@chromium.org, darin@chromium.org

Review URL: https://codereview.chromium.org/189903002

git-svn-id: svn://svn.chromium.org/blink/trunk@168867 bbb929c8-8fbe-4397-9dbb-9b2b20218538