Apply both phsical bounding box and clip to composited bounds.

third_party/WebKit/LayoutTests/compositing/layer-creation/fixed-position-change-out-of-view-in-view-expected.txt

 Layer tree when the fixed elements are out-of-view (should have just a root layer):
 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [785, 4016],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "LayoutBlockFlow (positioned) DIV id='fixed1'",
       "position": [100, -100],
       "bounds": [10, 10],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#C0C0C0"
     },
     {
       "name": "LayoutBlockFlow (positioned) DIV id='fixed2'",
       "position": [100, 100],
-      "bounds": [10, 0],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#C0C0C0"
     }
   ]
 }
 Layer tree when the fixed elements are in-view (both fixed elements should have layers):
 {
   "layers": [
     {
@@ skipping 33 matching lines @@
       "name": "LayoutBlockFlow (positioned) DIV id='fixed1'",
       "position": [100, -100],
       "bounds": [10, 10],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#C0C0C0"
     },
     {
       "name": "LayoutBlockFlow (positioned) DIV id='fixed2'",
       "position": [100, 100],
-      "bounds": [10, 0],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#C0C0C0"
     }
   ]
 }