[webkit-line-clamp] Ensure that a -w-l-c box creates a new FC.

As above - this fixes:
external/wpt/css/css-overflow/webkit-line-clamp-004.html

We weren't performing the correct relayout if the lines exactly matched
the requested number of lines.

Bug: 993813
Change-Id: I4ab3a702161bcb8e8cd10fc8ea6837931ebac16d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2224601
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774000}

third_party/blink/renderer/core/layout/layout_block_flow.cc

@@ -4330,6 +4330,7 @@ bool LayoutBlockFlow::CreatesNewFormattingContext() const {
       IsDocumentElement() || IsGridItem() || IsWritingModeRoot() ||
       IsMathItem() || StyleRef().Display() == EDisplay::kFlowRoot ||
       ShouldApplyPaintContainment() || ShouldApplyLayoutContainment() ||
+      StyleRef().IsDeprecatedWebkitBoxWithVerticalLineClamp() ||
       StyleRef().SpecifiesColumns() ||
       StyleRef().GetColumnSpan() == EColumnSpan::kAll) {
     // The specs require this object to establish a new formatting context.