Remove PositionFixedWithCompositedDescendants compositing reason.

The original CL that added it was:
https://codereview.chromium.org/1413363003

which has to do with the ways in which the pre-BGPT system explained
scrolling and fixed-position elements to the compositor. Since then
BGPT shipped, which no longer requires communicating this via the
graphics layer tree.

Bug: 1096732,950418

Change-Id: I9d6fe98298d69244ad4ca298e934d5c5765cf2da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2253282Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Auto-Submit: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780481}

third_party/blink/renderer/core/paint/compositing/compositing_requirements_updater.cc

@@ -183,11 +183,6 @@ static CompositingReasons SubtreeReasonsForCompositing(
     subtree_reasons |= CompositingReason::kIsolateCompositedDescendants;
   }
 
-  if (layer->GetLayoutObject().StyleRef().GetPosition() == EPosition::kFixed) {
-    subtree_reasons |=
-        CompositingReason::kPositionFixedWithCompositedDescendants;
-  }
-
   // A layer with preserve-3d or perspective only needs to be composited if
   // there are descendant layers that will be affected by the preserve-3d or
   // perspective.

third_party/blink/renderer/platform/graphics/compositing_reasons.cc

@@ -103,9 +103,6 @@ constexpr CompositingReasonStringMap kCompositingReasonsStringMap[] = {
     {CompositingReason::kIsolateCompositedDescendants,
      "isolateCompositedDescendants",
      "Should isolate descendants to apply a blend effect"},
-    {CompositingReason::kPositionFixedWithCompositedDescendants,
-     "positionFixedWithCompositedDescendants"
-     "Is a position:fixed element with composited descendants"},
     {CompositingReason::kRoot, "root", "Is the root layer"},
     {CompositingReason::kLayerForHorizontalScrollbar,
      "layerForHorizontalScrollbar",

third_party/blink/renderer/platform/graphics/compositing_reasons.h

@@ -63,7 +63,6 @@ using CompositingReasons = uint64_t;
   V(PerspectiveWith3DDescendants)                                             \
   V(Preserve3DWith3DDescendants)                                              \
   V(IsolateCompositedDescendants)                                             \
-  V(PositionFixedWithCompositedDescendants)                                   \
                                                                               \
   /* The root layer is a special case. It may be forced to be a layer, but it \
   also needs to be a layer if anything else in the subtree is composited. */  \

third_party/blink/web_tests/compositing/squashing/no-squashing-into-fixed-position-that-clips-expected.txt

@@ -6,13 +6,6 @@
       "contentsOpaque": true,
       "backgroundColor": "#FFFFFF"
     },
-    {
-      "name": "LayoutNGBlockFlow (positioned) DIV class='fixedpos'",
-      "position": [0, 50],
-      "bounds": [800, 550],
-      "contentsOpaque": true,
-      "backgroundColor": "#ADD8E6"
-    },
     {
       "name": "LayoutNGBlockFlow (positioned) DIV class='compositedlayer'",
       "bounds": [24, 100],

third_party/blink/web_tests/flag-specific/disable-layout-ng/compositing/squashing/no-squashing-into-fixed-position-that-clips-expected.txt

@@ -6,13 +6,6 @@
       "contentsOpaque": true,
       "backgroundColor": "#FFFFFF"
     },
-    {
-      "name": "LayoutBlockFlow (positioned) DIV class='fixedpos'",
-      "position": [0, 50],
-      "bounds": [800, 550],
-      "contentsOpaque": true,
-      "backgroundColor": "#ADD8E6"
-    },
     {
       "name": "LayoutBlockFlow (positioned) DIV class='compositedlayer'",
       "bounds": [24, 100],