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

third_party/WebKit/LayoutTests/platform/linux/fast/repaint/line-flow-with-floats-8-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [485, 0, 15, 600],
         [303, 420, 67, 19],
@@ skipping 73 matching lines @@
         "InlineTextBox 'stamping about, and'",
         "RootInlineBox",
         "InlineTextBox 'shouting \u2018Off with his head!\u2019 or \u2018Off with\n'",
         "InlineTextBox 'her head!\u2019 about'",
         "RootInlineBox",
         "InlineTextBox 'once in a minute.\n'",
         "RootInlineBox",
         "LayoutBlockFlow HTML",
         "LayoutBlockFlow P",
         "LayoutBlockFlow P",
+        "RootInlineBox",
         "LayoutText #text",
         "InlineTextBox 'difficult game indeed.\n'",
         "LayoutText #text",
         "InlineTextBox 'The players all played at once without waiting\n'",
         "LayoutText #text",
         "InlineTextBox 'for turns,'",
         "InlineTextBox 'quarrelling all the while, and fighting for the hedgehogs; and'",
         "InlineTextBox 'in\n'",
         "InlineTextBox 'a very short time '",
         "LayoutText #text",
@@ skipping 12 matching lines @@
         "InlineTextBox 'minute, \u2018and then,\u2019 thought she, \u2018what would become of\n'",
         "InlineTextBox 'me?'",
         "InlineTextBox 'They\u2019re dreadfully fond of beheading people here; the great'",
         "InlineTextBox 'wonder is, that there\u2018s any one left alive!\u2019'",
         "VerticalScrollbar"
       ]
     }
   ]
 }