The :clock_skew_tool binary is a tool for measuring the local device's clock skew. Specifically, the ClockSkewTool class drives the NetworkTimeTracker class, causing it to repeatedly fetch the current time from a time server, compare it to the system clock, and record results in histograms. On each iteration, ClockSkewTool prints a report containing the relevant histograms to stdout.
This tool was created to help investigate the distribution and privacy implications of clock skew. See the (design doc) and tracking bug: https://crbug.com/1258624.