Clean up needsLayout bit handling in RenderObject.

-s/needsLayout/selfNeedsLayout since RenderObject exposes a needsLayout
method that's actually the union of all the sub-needsLayout bits.
-Change forceLayout/forceChildLayout to set the bits directly instead
of calling the public method that walks up the containing block chain.
This gets rid of two more uses of MarkOnlyThis.

Review URL: https://chromiumcodereview.appspot.com/23064004

git-svn-id: svn://svn.chromium.org/blink/trunk@156340 bbb929c8-8fbe-4397-9dbb-9b2b20218538