[Layout API] Use LayoutViewItem in PaintPropertyTreeBuilderTest.cpp

There are two instances of document().layoutView(), which is deprecated.
I changed each to layoutViewItem() and added a method on LayoutItem
to proxy to the relevant method on LayoutObject.

There are no functional changes.

BUG=499321

Review-Url: https://codereview.chromium.org/2110893002
Cr-Commit-Position: refs/heads/master@{#402948}
2 files changed