Stop using VisiblePosition in DocumentMarkerController

This patch changes DocumentMarkerController to use VisiblePosition-
free versions of Start/EndOfWord, to reduce usage of VisiblePosition.

Bug: 657237
Change-Id: I344c98c9163ddc06f338e309a6b3e372d5fc2dda
Reviewed-on: https://chromium-review.googlesource.com/c/1330747
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608854}

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

@@ -397,13 +397,11 @@ DocumentMarker* DocumentMarkerController::FirstMarkerAroundPosition(
     return nullptr;
 
   const PositionInFlatTree start_of_word_or_null =
-      StartOfWord(CreateVisiblePosition(position), kPreviousWordIfOnBoundary)
-          .DeepEquivalent();
+      StartOfWordPosition(position, kPreviousWordIfOnBoundary);
   const PositionInFlatTree start =
       start_of_word_or_null.IsNotNull() ? start_of_word_or_null : position;
   const PositionInFlatTree end_of_word_or_null =
-      EndOfWord(CreateVisiblePosition(position), kNextWordIfOnBoundary)
-          .DeepEquivalent();
+      EndOfWordPosition(position, kNextWordIfOnBoundary);
   const PositionInFlatTree end =
       end_of_word_or_null.IsNotNull() ? end_of_word_or_null : position;