Compute style to avoid crashes for layout tree building.

We don't have a repro for this, but there are crashes in the wild where
we haven't got a ComputedStyle when attaching the layout tree for a UA
shadow DOM element inside an HTML input element.

Add a NOTREACHED to try to catch it, but fall back to compute the style
to avoid crashes when it happens.

Bug: 875796
Change-Id: I24ef78183df6faa6bfea88a7e5f8ad0d7f2bf14c
Reviewed-on: https://chromium-review.googlesource.com/1182806
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584690}

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

@@ -131,7 +131,13 @@ bool LayoutTreeBuilderForElement::ShouldCreateLayoutObject() const {
 }
 
 ComputedStyle& LayoutTreeBuilderForElement::Style() const {
-  DCHECK(style_);
+  if (!style_) {
+    // TODO(futhark@chromium.org): this should never happen, but we currently
+    // have crashes in the wild because of this (https://crbug.com/875796).
+    // Please report if you ever end up here.
+    NOTREACHED();
+    style_ = node_->StyleForLayoutObject();
+  }
   return *style_;
 }