Need to repaint its ::first-line background when a block moves.

third_party/WebKit/LayoutTests/fast/repaint/multicol-with-block-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [488, 8, 150, 20],
         [488, 8, 80, 20]
       ],
       "paintInvalidationClients": [
         "LayoutBlockFlow DIV id='target'",
+        "RootInlineBox",
         "LayoutText #text",
         "InlineTextBox 'PASS'"
       ]
     }
   ]
 }