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

third_party/WebKit/LayoutTests/platform/linux/fast/repaint/search-field-cancel-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [12, 47, 136, 16],
         [12, 47, 58, 16]
       ],
       "paintInvalidationClients": [
         "LayoutBlockFlow DIV id='inner-editor'",
+        "RootInlineBox",
         "LayoutText #text",
         "InlineTextBox 'some text'"
       ]
     }
   ]
 }