IDL compiler: ActivityLog for readonly attributes

Moving on to extended attributes, starting with ActivityLog.

I've also added brief documentation on the docs page;
is it accurate and useful?
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-ActivityLog-m-a-

Also rearranged v8_utilities.py, so the actual utilities are at the top, while "shared support for extended attributes" (shared across interfaces, attributes, and operations) is at the bottom, with "which Extended Attribute" clearly marked.

BUG=239771

Review URL: https://codereview.chromium.org/25689002

git-svn-id: svn://svn.chromium.org/blink/trunk@158818 bbb929c8-8fbe-4397-9dbb-9b2b20218538