[tools] Include full benchmark measurement name in error reporting

For benchmarks with multiple measurements (e.g. runtime, compile
time, generated code size), in case of error, we'd get only the
last portion of the name ("Compile"), which is ambiguous and makes
debugging dificult. This change includes the full name, e.g.
"Unity/Physics/Compile".

BUG=

Review-Url: https://codereview.chromium.org/2615623003
Cr-Commit-Position: refs/heads/master@{#42078}

tools/run_perf.py

@@ -179,7 +179,7 @@ class Measurement(object):
   gathered by repeated calls to ConsumeOutput.
   """
   def __init__(self, graphs, units, results_regexp, stddev_regexp):
-    self.name = graphs[-1]
+    self.name = '/'.join(graphs)
     self.graphs = graphs
     self.units = units
     self.results_regexp = results_regexp