[DoNotCheckSecurity=Getter] => [DoNotCheckSecurity] for readonly attribute

IDL cleanup.

Docs updated:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-DoNotCheckSecurity-m-a-

TBR=haraken

Review URL: https://codereview.chromium.org/77693002

git-svn-id: svn://svn.chromium.org/blink/trunk@162343 bbb929c8-8fbe-4397-9dbb-9b2b20218538