Version: 56.0.2896.0
OS: Android

What steps will reproduce the problem?
(1) Run the telemetry benchmark: tools/perf/run_benchmark blink_style.polymer --browser=android-chromium --story-filter="paper-button"

While tracking down failures in  issue 655627  I came across this unrelated failure. I've narrowed it down to m_private->document() being nullptr in WebSurroundingText::textContent(). This was added recently as a DCHECK and layout that causes the crash in release builds.

The root cause being that the VisiblePosition created in VisiblePositionTemplate<Strategy>::create hits the deepPosition.isNull() branch and returns a default constructed VisiblePosition which has no document.