Clip region based columns properly.

Need to call RenderRegion::overflowRectForFlowThreadPortion() first,
and THEN clip in the column gaps. overflowRectForFlowThreadPortion()
expands the logical left and right to the flow thread's overflow
rectangle, effectively defeating inline direction clipping completely.

This patch is ported from WebKit.
https://bugs.webkit.org/show_bug.cgi?id=118499
Reviewed in WebKit by David Hyatt.

BUG=269441

Review URL: https://chromiumcodereview.appspot.com/22577002

git-svn-id: svn://svn.chromium.org/blink/trunk@158387 bbb929c8-8fbe-4397-9dbb-9b2b20218538