Don't use stale compositing state in LocalFrame::nodeImage

This patch fixes a compositor chicken and egg bug in
LocalFrame::nodeImage where stale compositor state was read through
setBaseBackgroundColor. An updateLayoutAndStyleForPainting call has
been moved before setBaseBackgroundColor in nodeImage.

Additionally, two unnecessary protector RefPtrs have been removed
because the callers already ref'ed the objects. The Node RefPtr is
ref'ed by Clipboard, the nodeImage() caller. The LocalFrame RefPtr
is ref'ed by DragController::doSystemDrag.

As a cleanup, LocalFrame::nodeImage has been changed to
take a reference since the node cannot be null.

BUG=354373

Review URL: https://codereview.chromium.org/214053003

git-svn-id: svn://svn.chromium.org/blink/trunk@170135 bbb929c8-8fbe-4397-9dbb-9b2b20218538