Refactor BenchmarkResult.Add to take enum for data_type

tools/perf/measurements/tab_switching.py

Index: tools/perf/measurements/tab_switching.py
===================================================================
--- tools/perf/measurements/tab_switching.py	(revision 222891)
+++ tools/perf/measurements/tab_switching.py	(working copy)
@@ -17,6 +17,10 @@
 from telemetry.page import page_measurement
 from telemetry.page import page_runner
 
+# Get build scripts into our path.
+util.AddDirToPythonPath(util.GetChromiumSrcDir(), 'build', 'util', 'lib')
+from common import perf_result_data_type # pylint: disable=F0401
+
 # TODO: Revisit this test once multitab support is finalized.
 
 class TabSwitching(page_measurement.PageMeasurement):
@@ -84,4 +88,4 @@
         first_histogram)
 
     results.AddSummary(histogram_name, '', diff_histogram,
-        data_type='unimportant-histogram')
+                       data_type=perf_result_data_type.UNIMPORTANT_HISTOGRAM)