Promote fixed: position elements with composited descendants even if they don't scroll

This is effectively a partial revert of
9b503962 done manually to avoid
reverting de4c1142 which is necessary
for sticky elements.

Bug=742213
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2977273002
Cr-Commit-Position: refs/heads/master@{#487281}

third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp

@@ -194,8 +194,11 @@ 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 |=
           kCompositingReasonPositionFixedOrStickyWithCompositedDescendants;