[Telemetry] Specify units for tab switching time.

BUG=415564

Review URL: https://codereview.chromium.org/586503002

Cr-Commit-Position: refs/heads/master@{#295666}

tools/perf/measurements/tab_switching.py

@@ -96,6 +96,6 @@ class TabSwitching(page_test.PageTest):
         first_histogram)
 
     results.AddSummaryValue(
-        histogram.HistogramValue(None, display_name, '',
+        histogram.HistogramValue(None, display_name, 'ms',
                                  raw_value_json=diff_histogram,
                                  important=False))