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

third_party/WebKit/LayoutTests/platform/linux/fast/repaint/positioned-document-element-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#008000",
       "repaintRects": [
         [108, 116, 402, 20],
         [108, 116, 402, 20],
         [108, 116, 402, 19],
         [100, 100, 418, 52],
         [8, 16, 784, 20],
         [8, 16, 784, 20],
         [8, 16, 402, 19],
         [0, 0, 800, 600],
         [0, 0, 800, 52]
       ],
       "paintInvalidationClients": [
         "RootInlineBox",
         "InlineTextBox 'Tests that the entire viewport is painted with a floated html element.'",
         "LayoutView #document",
         "LayoutBlockFlow (positioned) HTML class='changed'",
         "LayoutBlockFlow BODY",
         "LayoutBlockFlow P",
+        "RootInlineBox",
         "LayoutText #text",
         "InlineTextBox 'Tests that the entire viewport is painted with a floated html element.'"
       ]
     }
   ]
 }