IDL compiler: Simplify [Replaceable] handling by banning [Custom(Setter), Replaceable]

[Custom]/[CustomSetter] and [Replaceable] do not interact well,
and cause significant complexity in the CG.
(See discussion at CL below for to see how ugly and confusing it is.)

They are used for only 2 attributes in Window.idl, and this can be fixed
by removing "[Replaceable] readonly" (since they have actual custom setters).

Docs also updated to say as much
(i.e., "[Custom]/[CustomSetter] and [Replaceable] are incompatible").

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

Followup to:
IDL compiler: [Custom(Setter), Replaceable]
https://codereview.chromium.org/35123002/

BUG=239771

Review URL: https://codereview.chromium.org/36553002

git-svn-id: svn://svn.chromium.org/blink/trunk@160420 bbb929c8-8fbe-4397-9dbb-9b2b20218538