Push through multiple regions when an object doesn't fit in any of them. Unlike with pages and
columns, when regions have a non-uniform height, we may need to push through multiple regions
in order to find one that fits.
        
Added an optimization for quickly noticing if regions do have a uniform height so that we can
treat them like columns and pages if so.

Also fixed the end line matchup to properly null out endLine when no next line box exists. The new
layout tests I wrote to cover this feature exposed this crasher, so fixing it in order to land
the new tests.

Reviewed by Anders Carlsson.

Source/WebCore: 

Added new tests in fast/regions.

* rendering/RenderBlock.cpp:
(WebCore::RenderBlock::adjustForUnsplittableChild):
(WebCore::RenderBlock::pushToNextPageWithMinimumLogicalHeight):
(WebCore::RenderBlock::adjustLinePositionForPagination):
* rendering/RenderBlock.h:
* rendering/RenderBlockLineLayout.cpp:
(WebCore::RenderBlock::matchedEndLine):
* rendering/RenderFlowThread.cpp:
(WebCore::RenderFlowThread::RenderFlowThread):
(WebCore::RenderFlowThread::layout):
* rendering/RenderFlowThread.h:

LayoutTests: 

* fast/regions/webkit-flow-float-pushed-to-last-region.html: Added.
* fast/regions/webkit-flow-float-unable-to-push.html: Added.
* platform/mac/fast/regions/webkit-flow-float-pushed-to-last-region-expected.png: Added.
* platform/mac/fast/regions/webkit-flow-float-pushed-to-last-region-expected.txt: Added.
* platform/mac/fast/regions/webkit-flow-float-unable-to-push-expected.png: Added.
* platform/mac/fast/regions/webkit-flow-float-unable-to-push-expected.txt: Added.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@95855 268f45cc-cd09-0410-ab3c-d52691b4dbfc