Only reattach whitespace siblings when going from no renderer to having one.

We only need to reattach whitespace siblings when we go from having no
renderer to having one. This avoids an n^2 traversal of siblings when
attaching lots of display: none or whitespace (without a renderer) siblings.

This brings us inline with Safari for the whitespace benchmark in the bug,
except for the Shadow DOM overhead that we have due to both Safari and Chrome
having an n^2 in Text::textRendererIsNeeded due to calling
NodeRenderingTraversal::previousSiblingRenderer. If we fix that then we can
run the benchmark in 4ms.

BUG=399816

Review URL: https://codereview.chromium.org/451893002

git-svn-id: svn://svn.chromium.org/blink/trunk@179780 bbb929c8-8fbe-4397-9dbb-9b2b20218538