RenderGrid::paintChild() declared but not defined.
RenderGrid::paintChild() declared but not defined.

https://codereview.chromium.org/594293002/ moved paint code from RenderGrid
to GridPainter, including the implementation of paintChild(), but the
declaration was left behind in RenderGrid.

R=chrishtr@chromium.org

Review URL: https://codereview.chromium.org/601113002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@182745 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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