Detach ::first-letter in Element::DetachLayoutTree.

The code for first-letter was recently reworked, but this was overlooked.

Fixes DCHECK failures for LayoutNG.

R=futhark@chromium.org, yosin@chromium.org

Bug: 872207
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I03e7aa3216c3e6c9e8576ff2d76cdb20730a9aae
Reviewed-on: https://chromium-review.googlesource.com/1166921Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581813}

third_party/WebKit/LayoutTests/FlagExpectations/enable-blink-features=LayoutNG

@@ -70,11 +70,8 @@ crbug.com/591099 css3/flexbox/intrinsic-width-orthogonal-writing-mode.html [ Fai
 crbug.com/591099 css3/flexbox/line-wrapping.html [ Failure ]
 crbug.com/591099 css3/flexbox/scrollbars-auto.html [ Failure ]
 crbug.com/714962 css3/masking/clip-path-reference-box-inline.html [ Failure ]
-crbug.com/591099 editing/inserting/insert-character-in-first-letter-crash.html [ Crash ]
 crbug.com/591099 editing/selection/continuations-with-move-caret-to-boundary.html [ Failure Pass ]
 crbug.com/591099 editing/selection/paint-hyphen.html [ Pass ]
-crbug.com/591099 editing/text-iterator/first-letter-word-boundary.html [ Crash ]
-crbug.com/591099 editing/text-iterator/read-past-cloned-first-letter.html [ Crash ]
 crbug.com/591099 external/wpt/2dcontext/drawing-images-to-the-canvas/drawimage_html_image_11.html [ Pass ]
 crbug.com/591099 external/wpt/WebCryptoAPI/generateKey/failures.worker.html [ Timeout ]
 crbug.com/591099 external/wpt/WebCryptoAPI/generateKey/failures_AES-CBC.worker.html [ Timeout ]
@@ -423,10 +420,6 @@ crbug.com/591099 fast/css-intrinsic-dimensions/height-positioned.html [ Failure
 crbug.com/807708 fast/css-intrinsic-dimensions/width-avoid-floats.html [ Failure ]
 crbug.com/591099 fast/css/absolute-inline-alignment-2.html [ Pass ]
 crbug.com/591099 fast/css/case-transform.html [ Failure ]
-crbug.com/591099 fast/css/first-letter-block-form-controls-crash.html [ Crash ]
-crbug.com/591099 fast/css/first-letter-capitalized-edit-select-crash.html [ Crash ]
-crbug.com/591099 fast/css/first-letter-crash-document-disposal.html [ Crash ]
-crbug.com/591099 fast/css/first-letter-text-fragment-crash.html [ Crash ]
 crbug.com/835484 fast/css/focus-ring-continuations.html [ Failure ]
 crbug.com/835484 fast/css/focus-ring-recursive-continuations.html [ Failure ]
 crbug.com/835484 fast/css/focus-ring-recursive-inlines.html [ Failure ]
@@ -449,7 +442,6 @@ crbug.com/591099 fast/overflow/overflow-update-transform.html [ Failure ]
 crbug.com/591099 fast/overflow/recompute-overflow-of-layout-root-container.html [ Failure ]
 crbug.com/591099 fast/replaced/table-replaced-element.html [ Failure ]
 crbug.com/591099 fast/ruby/position-after.html [ Failure ]
-crbug.com/591099 fast/ruby/ruby-first-letter.html [ Crash ]
 crbug.com/591099 fast/scrolling/content-box-smaller-than-scrollbar.html [ Failure ]
 crbug.com/591099 fast/scrolling/jquery-rtl-scroll-type.html [ Failure ]
 crbug.com/591099 fast/scrolling/scrollbar-tickmarks-hittest.html [ Failure Pass ]

third_party/blink/renderer/core/dom/element.cc

@@ -2189,6 +2189,7 @@ void Element::DetachLayoutTree(const AttachContext& context) {
 
   DetachPseudoElement(kPseudoIdAfter, context);
   DetachPseudoElement(kPseudoIdBackdrop, context);
+  DetachPseudoElement(kPseudoIdFirstLetter, context);
 
   if (!context.performing_reattach && IsUserActionElement()) {
     if (IsHovered())