[CSS Regions] In a region chain with auto-height regions, lines get their length based only on the first region
https://bugs.webkit.org/show_bug.cgi?id=118531

Reviewed by Alexandru Chiculita.

Source/WebCore:

When computing the height a flow thread it's possible to overflow the maximum LayoutUnit and obtain a negative value.
This leads to invalid results during layout when computing the region range and the RenderBoxRegion info for the
descendant boxes of the flow thread.

This issue appears especially during the auto-height algorithm because it initializes the auto-height regions
height with the LayoutUnit::max() / 2 value. Summing two such regions overflows and results in a negative value.

The fix clamps the maximum height of the flow thread to LayoutUnit::max() / 2. This doesn't affect the auto-height
algorithm because regionAtBlockOffset() will still return the correct values as the auto-height regions content
is established and their height updated.

Tests: fast/regions/autoheight-correct-region-for-lines-2.html
       fast/regions/autoheight-correct-region-for-lines.html

* rendering/RenderBlock.cpp:
(WebCore::RenderBlock::updateRegionsAndShapesBeforeChildLayout):
* rendering/RenderFlowThread.cpp:
(WebCore::RenderFlowThread::computeLogicalHeight):
(WebCore::RenderFlowThread::setRegionRangeForBox):
(WebCore::RenderFlowThread::updateRegionsFlowThreadPortionRect):
* rendering/RenderFlowThread.h:
* rendering/RenderRegion.cpp:
(WebCore::RenderRegion::maxPageLogicalHeight):

LayoutTests:

Add tests verifying the lines widths are correctly computed in auto-height regions with and without a max-height set.

* fast/regions/autoheight-correct-region-for-lines-2-expected.html: Added.
* fast/regions/autoheight-correct-region-for-lines-2.html: Added.
* fast/regions/autoheight-correct-region-for-lines-expected.html: Added.
* fast/regions/autoheight-correct-region-for-lines.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@152572 268f45cc-cd09-0410-ab3c-d52691b4dbfc