Work around reversed word range in DocumentMarkerController::FirstMarkerAroundPosition

There are some crashes in the function due to legacy implemetation of
StartOfWord() creating invalid/legacy positions, which lead to reversed
word ranges.

This patch adds handling of reversed word ranges so that we don't crash,
before switching StartOfWord() to proper implementation.

Bug: 886589
Change-Id: I8ad988247c6e4ab4a4a7cf90675d7df5f326f56f
Reviewed-on: https://chromium-review.googlesource.com/c/1298018Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602474}

third_party/blink/renderer/core/editing/markers/document_marker_controller.cc

@@ -407,7 +407,13 @@ DocumentMarker* DocumentMarkerController::FirstMarkerAroundPosition(
   const PositionInFlatTree end =
       end_of_word_or_null.IsNotNull() ? end_of_word_or_null : position;
 
-  DCHECK_LE(start, end) << "|start| should be before |end|.";
+  if (start > end) {
+    // TODO(crbug.com/778507): We shouldn't reach here, but currently do due to
+    // legacy implementation of StartOfWord(). Rewriting StartOfWord() with
+    // TextOffsetMapping should fix it.
+    NOTREACHED() << "|start| should be before |end|.";
+    return nullptr;
+  }
 
   const Node* const start_node = start.ComputeContainerNode();
   const unsigned start_offset = start.ComputeOffsetInContainerNode();