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

third_party/WebKit/LayoutTests/platform/linux/fast/repaint/line-flow-with-floats-2-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [485, 0, 15, 600],
         [372, 384, 48, 81],
@@ skipping 26 matching lines @@
         "InlineTextBox 'straightened\n'",
         "InlineTextBox 'out, and was going to give the hedgehog a'",
         "RootInlineBox",
         "InlineTextBox 'blow with its head, it\n'",
         "InlineTextBox ''",
         "InlineTextBox ' twist itself round and look up in her'",
         "RootInlineBox",
         "LayoutBlockFlow HTML",
         "LayoutBlockFlow P",
         "LayoutBlockFlow P",
+        "RootInlineBox",
         "LayoutText #text",
         "InlineTextBox 'The chief difficulty Alice found at first was in managing her'",
         "InlineTextBox 'flamingo: she succeeded in getting its body tucked away,'",
         "InlineTextBox 'comfortably enough, under her arm, with its legs hanging'",
         "InlineTextBox 'down,\n'",
         "InlineTextBox 'but generally, just as she had got its neck nicely'",
         "InlineTextBox 'straightened\n'",
         "InlineTextBox 'out, and was going to give the hedgehog a'",
         "InlineTextBox 'blow with its head, it\n'",
         "LayoutInline I id='would'",
@@ skipping 47 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"
       ]
     }
   ]
 }