RenderBlock::isSelfCollapsingBlock() should early exit on a relayout boundary

RenderBox::layoutOverflowRectForPropagation() needs to know whether the current
block is self-collapsing to pad layout overflow with after-margin. This can
trigger an assertion failure that it expects clean layout. This breaks the
contract of relayout boundary that its internal layout shouldn't matter to the
parent.

This CL simply reorders the assertion so we can give a quick answer if the box
is a relayout boundary before we assume clean layout.

This CL fixes an existing test failure cause by another CL.
CL affected by this bug: https://codereview.chromium.org/203463007

TEST=fast/overflow/overflow-dirty-relayout-boundary-no-crash.html
BUG=365507

Review URL: https://codereview.chromium.org/255433003

git-svn-id: svn://svn.chromium.org/blink/trunk@172577 bbb929c8-8fbe-4397-9dbb-9b2b20218538