Revert "Don't promote position: fixed elements with composited descendants if they don't scroll"

This reverts commit 9ca72e1a.

Reason for revert: Suspected cause of crbug.com/863361 - it seems that scrolling the viewport is still possible.

Bug: 715699, 863361

Original change's description:
> Don't promote position: fixed elements with composited descendants if they don't scroll
> 
> This is a re-land of an old CL, which was reverted in https://codereview.chromium.org/2977273002
> due to a WebView issue. We believe the underlying issue may have been resolved, so should try
> re-landing this.
> 
> Bug: 715699
> Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
> Change-Id: I0da6ab6f55d0fb4ba0760378f182217e8c4546c1
> Reviewed-on: https://chromium-review.googlesource.com/1112387
> Reviewed-by: Robert Flack <flackr@chromium.org>
> Commit-Queue: Robert Flack <flackr@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#571143}

TBR=flackr@chromium.org,smcgruer@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 715699
Change-Id: I6a9e969a4118b75819dcd297d18d9a2ae1c9abca
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Reviewed-on: https://chromium-review.googlesource.com/1179921Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584087}

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

@@ -189,8 +189,11 @@ static CompositingReasons SubtreeReasonsForCompositing(
   // We ignore LCD text here because we are required to composite
   // scroll-dependant fixed position elements with composited descendants for
   // correctness - even if we lose LCD.
+  //
+  // TODO(smcgruer): Only composite fixed if needed (http://crbug.com/742213)
   const bool ignore_lcd_text = true;
-  if (compositing_reason_finder.RequiresCompositingForScrollDependentPosition(
+  if (layer->GetLayoutObject().Style()->GetPosition() == EPosition::kFixed ||
+      compositing_reason_finder.RequiresCompositingForScrollDependentPosition(
           layer, ignore_lcd_text)) {
     subtree_reasons |=
         CompositingReason::kPositionFixedOrStickyWithCompositedDescendants;