[New Multicolumn] Create RenderMultiColumnSet during renderer creation, not during layout.

This allowed for removal of 2 parameters to regionAtBlockOffset(), and
we can also make it const now.

Also make some preparations for multiple RenderMultiColumnSet
objects. This is required by column-span:all.

Add some generic multicol documentation in front of the class definition
of RenderMultiColumnFlowThread, since that's the main class for multicol.
Also improved RenderMultiColumnSet documentation somewhat.

All this is just code improvements; no behavioral changes intended.

Review URL: https://codereview.chromium.org/246403015

git-svn-id: svn://svn.chromium.org/blink/trunk@172698 bbb929c8-8fbe-4397-9dbb-9b2b20218538