Merge [CheckSecurity] and [CheckSecurityForNode]

[CheckSecurity] => [CheckSecurity=Frame]
[CheckSecurityForNode] => [CheckSecurity=Node]
(For clarity.)

Note that in the code, I'm not checking that the correct Frame/Node value is given,
since only one value is possible for a given kind.
(Frame on interface, Node on method/attribute.)
However, the IDL extended attribute validator does check
that it's Frame or Node, and the specific value is useful
for documentation in the IDL (specifies what's being checked).

Updated docs:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-CheckSecurity-i-m-a-

Review URL: https://codereview.chromium.org/68203009

git-svn-id: svn://svn.chromium.org/blink/trunk@161777 bbb929c8-8fbe-4397-9dbb-9b2b20218538