Have Document::nodeWillBeRemoved() take a reference

Have Document::nodeWillBeRemoved() take a reference instead of a pointer
as it can never be null. This makes the caller simpler as it already
have a Node reference. This also gets rid of several unnecessary null
checks.

R=adamk

Review URL: https://codereview.chromium.org/48803004

git-svn-id: svn://svn.chromium.org/blink/trunk@160986 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move FloatWithRect to RenderBlockFlow

FloatWithRect is only used by RenderBlockLineLayout, all of the logic
of which belongs attributed to RenderBlockFlow.

BUG=302024

Review URL: https://codereview.chromium.org/53533002

git-svn-id: svn://svn.chromium.org/blink/trunk@160987 bbb929c8-8fbe-4397-9dbb-9b2b20218538