IDL compiler: CallWith for getters

In general CallWith has 6 possible values, but for readonly attributes only
[CallWith=ScriptExecutionContext] occurs.

[CallWith=ScriptState] *could* occur, but is not used for attributes in any
real IDL files, so I'm omitting it.

ActiveWindow and FirstWindow occur in SetterCallWith, so we'll do it when
we're doing setters.

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

Style note:
Per recent use of 'include' as a mutable parameter (adding includes
is a side effect, when the function primarily does a computation),
I'm doing the same here with 'contents'.
This avoids having to return and unpack 3 values
(main value - here call_with_arguments - plus contents and includes)
and declaring and returning local variables for contents and includes.
It does make the function call slightly longer, but this
does make explicit that we're modifying these values.
(I think this is clearer.)

Also minor cleanup/simplification to some code by alancutter@,
and adding comments in templates to make grepping easier.

BUG=239771

Review URL: https://codereview.chromium.org/26209002

git-svn-id: svn://svn.chromium.org/blink/trunk@159008 bbb929c8-8fbe-4397-9dbb-9b2b20218538