RenderLayerRepainter::setBackingNeedsRepaintInRect crashes

We can issue a repaint while GraphicsLayerUpdater is walking the composited
layer mapping creating the GraphicsLayer tree because
GraphicsLayerUpdater::update calls updateRequiresOwnBackingStoreForAncestorReasons,
which can issue a repaint. Unfortunately, if the paint and tree orders are
arranged in a certain way, we can attempt to repaint into a squashing graphics
layer that has not yet been created.

The real solution to this problem is to separate issuing the repaints from
creating the new GraphicsLayer tree. Presumably we should issue repaints while
we still have the prior GraphicsLayer tree. However, in the near term, we can
just add a null check to avoid issuing repaints on non-existent GraphicsLayers.

R=esprehn@chromium.org
BUG=366384

Review URL: https://codereview.chromium.org/250763002

git-svn-id: svn://svn.chromium.org/blink/trunk@172571 bbb929c8-8fbe-4397-9dbb-9b2b20218538