Merge "Check for null surface pointer in renderOverdraw"

    // Set up the overdraw canvas.

    SkImageInfo offscreenInfo = SkImageInfo::MakeA8(surface->width(), surface->height());

    sk_sp<SkSurface> offscreen = surface->makeSurface(offscreenInfo);

    LOG_ALWAYS_FATAL_IF(!offscreen, "Failed to create offscreen SkSurface for overdraw viz.");

    SkOverdrawCanvas overdrawCanvas(offscreen->getCanvas());

    // Fake a redraw to replay the draw commands.  This will increment the alpha channel