Fix issue when background changes painting target layer

third_party/WebKit/LayoutTests/compositing/squashing/squash-above-fixed-1-expected.txt

 The gray div is a composited fixed-position element, and the cyan/lime elements should be squashed together on top. When scrolling, paragraphs may pop in-and-out of the squashing layer when they change overlapping status with respect to the composited layer underneath.
 
 This scenario tests (1) that content repaints correctly as layers pop in and out of squashing, and (2) that the positioning of the squashing layer remains correct (i.e. scrolls properly) when the squashing layer is on top of a fixed-position composited layer.
 
 CASE 1, original layer tree:
 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [785, 1400],
@@ skipping 80 matching lines @@
       "name": "LayoutBlockFlow (relative positioned) DIV id='paragraph-c' class='overlapping cyan'",
       "position": [0, 200],
       "bounds": [200, 100],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#00FFFF",
       "paintInvalidations": [
         {
           "object": "LayoutBlockFlow (relative positioned) DIV id='paragraph-c' class='overlapping cyan'",
           "rect": [0, 0, 200, 100],
+          "reason": "full"
+        },
+        {
+          "object": "LayoutBlockFlow (relative positioned) DIV id='paragraph-c' class='overlapping cyan'",
+          "rect": [0, 0, 200, 100],
           "reason": "compositing update"
         }
       ]
     },
     {
       "name": "Squashing Layer (first squashed layer: LayoutBlockFlow (relative positioned) DIV id='paragraph-d' class='overlapping lime')",
       "position": [0, 300],
       "bounds": [200, 300],
       "drawsContent": true,
       "paintInvalidations": [
@@ skipping 143 matching lines @@
       "name": "LayoutBlockFlow (relative positioned) DIV id='paragraph-d' class='overlapping lime'",
       "position": [0, 300],
       "bounds": [200, 100],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#00FF00",
       "paintInvalidations": [
         {
           "object": "LayoutBlockFlow (relative positioned) DIV id='paragraph-d' class='overlapping lime'",
           "rect": [0, 0, 200, 100],
+          "reason": "full"
+        },
+        {
+          "object": "LayoutBlockFlow (relative positioned) DIV id='paragraph-d' class='overlapping lime'",
+          "rect": [0, 0, 200, 100],
           "reason": "compositing update"
         }
       ]
     },
     {
       "name": "Squashing Layer (first squashed layer: LayoutBlockFlow (relative positioned) DIV id='paragraph-e' class='overlapping cyan')",
       "position": [0, 400],
       "bounds": [200, 300],
       "drawsContent": true,
       "paintInvalidations": [
@@ skipping 48 matching lines @@
       "object": "LayoutBlockFlow (relative positioned) DIV id='paragraph-f' class='overlapping lime'",
       "reason": "compositing update"
     },
     {
       "object": "LayoutBlockFlow (relative positioned) DIV id='paragraph-g' class='overlapping cyan'",
       "reason": "compositing update"
     }
   ]
 }