With http://crrev/c/1102157, we found that the expected results of following tests are not correct.
1) editing/execCommand/delete-non-editable-range-crash.html
2) editing/execCommand/format-block-contenteditable-false.html

Possibly because MostBackwardCaretPosition() isn't working as intended.

For one example:

Selection from EditingBoundaryAdjuster::AdjustSelection() is:

<div contenteditable>
  <h1><i>^foo</i><br><i>baz</i></h1>|
  <div contenteditable="false">bar</div>
</div>

In the later code, it will be adjusted to
<div contenteditable>
  <h1><i>^foo</i><br><i>baz</i></h1>
  <div contenteditable="false">|bar</div>
</div>