Add DCHECK() to NextWordPositionInternal() not to accept null-position

Because of |NextWordPositionInternal()| doesn't accept null-position, this
patch adds |DCHECK()| to catch such callers for ease of debugging.

Bug: 832061
Change-Id: Id987553c939db8f061e58eca09d7ebcd946482c9
Reviewed-on: https://chromium-review.googlesource.com/1013642Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551531}

third_party/blink/renderer/core/editing/visible_units_word.cc

@@ -81,6 +81,7 @@ PositionTemplate<Strategy> EndOfWordAlgorithm(
 
 PositionInFlatTree NextWordPositionInternal(
     const PositionInFlatTree& position) {
+  DCHECK(position.IsNotNull());
   PositionInFlatTree block_end_position;
   for (const LayoutBlock* block =
            &TextOffsetMapping::ComputeContainigBlock(position);