Utilize SelectionTemplate::ComputeRange() in CreateVisibleSelection()

This patch changes |CreateVisibleSelection()| to utilize |ComputeRange()| of
|SelectionTemplate| to simplify source code for improving code health.

Change-Id: I3d19d963cf04acba7f7bb0cb090dd45139f914a4
Reviewed-on: https://chromium-review.googlesource.com/897290Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533997}

third_party/WebKit/Source/core/editing/VisibleSelection.cpp

@@ -270,9 +270,8 @@ static SelectionTemplate<Strategy> ComputeVisibleSelection(
   const SelectionTemplate<Strategy>& editing_adjusted_selection =
       SelectionAdjuster::AdjustSelectionToAvoidCrossingEditingBoundaries(
           shadow_adjusted_selection);
-  const EphemeralRangeTemplate<Strategy> editing_adjusted_range(
-      editing_adjusted_selection.ComputeStartPosition(),
-      editing_adjusted_selection.ComputeEndPosition());
+  const EphemeralRangeTemplate<Strategy> editing_adjusted_range =
+      editing_adjusted_selection.ComputeRange();
   // TODO(editing-dev): Implement
   // const SelectionTemplate<Strategy>& adjusted_selection =
   // AdjustSelectionType(editing_adjusted_range);