[CSS Shapes] First line gets incorrectly adjusted in shape-inside due to rounding

In order to get consistent results of the first fit position of the content in shapes,
firstIncludedIntervalLogicalTop should take a FloatSize rather than LayoutSize for
minLogicalIntervalSize, because LayoutSize clamps the float value to int, when subpixel-layout
is disabled, thus firstIncludedIntervalLogicalTop could end up calculating with an unprecize
value. This change modifies firstIncludedIntervalLogicalTop to take FloatSize consistently.

Merged WebKit revision: http://trac.webkit.org/changeset/161604

BUG=333423

Review URL: https://codereview.chromium.org/133123003

git-svn-id: svn://svn.chromium.org/blink/trunk@164923 bbb929c8-8fbe-4397-9dbb-9b2b20218538