IDL compiler: anonymous indexed property getters

This starts special operations.
I've kept this simple, but I've made it generic, as we know
we'll also have indexed property setters, named property getters, etc.

For now I'm putting the tests in a separate file,
since these are interface-wide features, so I don't need to worry
about interface-wide extended attributes changing things.
Once I've implemented these, I'll merge them into the existing
TestInterfacePython tests to reduce the number of test results.

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/148083004

git-svn-id: svn://svn.chromium.org/blink/trunk@165894 bbb929c8-8fbe-4397-9dbb-9b2b20218538