Rename [EnabledAtRuntime] and [EnabledPerContext] in test support IDLs

Renames extended attributes in test support, from CL:
https://codereview.chromium.org/24053003
Also remove [RuntimeEnabled, PerContextEnabled] dual use from test,
as does not appear in real IDLs.

NOTRY=true

Review URL: https://codereview.chromium.org/27415005

git-svn-id: svn://svn.chromium.org/blink/trunk@159720 bbb929c8-8fbe-4397-9dbb-9b2b20218538