[Line Layout API] Change VisibleUnitsTest to use lineLayoutItem()

This is part of the ongoing refactoring to make clients use the line
layout API instead of poking at LayoutObjects directly.

No functional changes.

BUG=499321

Review URL: https://codereview.chromium.org/1624593003

Cr-Commit-Position: refs/heads/master@{#371738}

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

@@ -41,7 +41,7 @@ std::ostream& operator<<(std::ostream& ostream, const InlineBoxPosition& inlineB
 {
     if (!inlineBoxPosition.inlineBox)
         return ostream << "null";
-    return ostream << inlineBoxPosition.inlineBox->layoutObject().node() << "@" << inlineBoxPosition.offsetInBox;
+    return ostream << inlineBoxPosition.inlineBox->lineLayoutItem().node() << "@" << inlineBoxPosition.offsetInBox;
 }
 
 class VisibleUnitsTest : public EditingTestBase {