[CSS Regions][CSS Shapes] Update updateShapeAndSegmentsForCurrentLineInFlowThread to deal better with the first lines

We have a complex condition in updateShapeAndSegmentsForCurrentLineInFlowThread, which is
adjusting the first line to the shape's top. This patch adds two boolean to make that clear.

Merged WebKit revision: http://trac.webkit.org/changeset/158015

Review URL: https://codereview.chromium.org/50263005

git-svn-id: svn://svn.chromium.org/blink/trunk@160874 bbb929c8-8fbe-4397-9dbb-9b2b20218538
RefPtr should not be fast allocated

RefPtr should not be fast allocated in particular and it should not be allocated on
heap in general (except placement 'new' cases, as placement 'new' is used by WTF
containers).

Review URL: https://codereview.chromium.org/50493006

git-svn-id: svn://svn.chromium.org/blink/trunk@160875 bbb929c8-8fbe-4397-9dbb-9b2b20218538