Version: ToT at r508367

Repro in a unit test:

TEST_F(VisibleUnitsTest,
       MostBackwardCaretPositionInRemainingTextWithLeadingCollapsedSpace) {
  InsertStyleElement(
      "div {white-space: pre-line}"
      "div::first-letter {color:red}");
  SetBodyContent("<div>F \nbar</div>");
  const Node* text = GetDocument().QuerySelector("div")->firstChild();

  EXPECT_EQ(Position(text, 1), MostBackwardCaretPosition(Position(text, 2)));
} 

However, current implementation returns text@2.

The root cause is that, CanBeBackwardCaretPosition() assumes that the remaining text always starts with non-collapsed character, which is incorrect as in our example.