DCHECK validity of |caret_offset| in AdjustInlineBoxPositionForTextDirectionInternal()

The function accepts |caret_offset| at either left or right edge of
|inline_box|. This patch adds DCHECKs to ensure it.

Bug: 822575
Change-Id: I247361097f37f0cd9922442ea8c050f88136d9fc
Reviewed-on: https://chromium-review.googlesource.com/999021Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548695}

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

@@ -165,6 +165,9 @@ InlineBoxPosition AdjustInlineBoxPositionForTextDirectionInternal(
     InlineBox* inline_box,
     int caret_offset,
     UnicodeBidi unicode_bidi) {
+  DCHECK(caret_offset == inline_box->CaretLeftmostOffset() ||
+         caret_offset == inline_box->CaretRightmostOffset());
+
   const TextDirection primary_direction =
       inline_box->Root().Block().Style()->Direction();
   if (inline_box->Direction() == primary_direction)