[Tooling] report.py: don't crash on BigO/RMS benchmarks

Run into this by accident while writing benchmark to validate
the fix for https://bugs.llvm.org/show_bug.cgi?id=40965

Fixes #779.

tools/gbench/Inputs/test1_run1.json

@@ -97,6 +97,23 @@
       "real_time": 1,
       "cpu_time": 1,
       "time_unit": "s"
+    },
+    {
+      "name": "MyComplexityTest_BigO",
+      "run_name": "MyComplexityTest",
+      "run_type": "aggregate",
+      "aggregate_name": "BigO",
+      "cpu_coefficient": 4.2749856294592886e+00,
+      "real_coefficient": 6.4789275289789780e+00,
+      "big_o": "N",
+      "time_unit": "ns"
+    },
+    {
+      "name": "MyComplexityTest_RMS",
+      "run_name": "MyComplexityTest",
+      "run_type": "aggregate",
+      "aggregate_name": "RMS",
+      "rms": 4.5097802512472874e-03
     }
   ]
 }

tools/gbench/Inputs/test1_run2.json

@@ -97,6 +97,23 @@
       "real_time": 1,
       "cpu_time": 1,
       "time_unit": "ns"
+    },
+    {
+      "name": "MyComplexityTest_BigO",
+      "run_name": "MyComplexityTest",
+      "run_type": "aggregate",
+      "aggregate_name": "BigO",
+      "cpu_coefficient": 5.6215779594361486e+00,
+      "real_coefficient": 5.6288314793554610e+00,
+      "big_o": "N",
+      "time_unit": "ns"
+    },
+    {
+      "name": "MyComplexityTest_RMS",
+      "run_name": "MyComplexityTest",
+      "run_type": "aggregate",
+      "aggregate_name": "RMS",
+      "rms": 3.3128901852342174e-03
     }
   ]
 }

tools/gbench/report.py

@@ -114,6 +114,10 @@ def intersect(list1, list2):
     return [x for x in list1 if x in list2]
 
 
+def is_potentially_comparable_benchmark(x):
+    return ('time_unit' in x and 'real_time' in x and 'cpu_time' in x)
+
+
 def partition_benchmarks(json1, json2):
     """
     While preserving the ordering, find benchmarks with the same names in
@@ -125,10 +129,17 @@ def partition_benchmarks(json1, json2):
     names = intersect(json1_unique_names, json2_unique_names)
     partitions = []
     for name in names:
+        time_unit = None
         # Pick the time unit from the first entry of the lhs benchmark.
-        time_unit = (x['time_unit']
-                     for x in json1['benchmarks'] if x['name'] == name).next()
+        # We should be careful not to crash with unexpected input.
+        for x in json1['benchmarks']:
+            if (x['name'] == name and is_potentially_comparable_benchmark(x)):
+                time_unit = x['time_unit']
+                break
+        if time_unit is None:
+            break
         # Filter by name and time unit.
+        # All the repetitions are assumed to be comparable.
         lhs = [x for x in json1['benchmarks'] if x['name'] == name and
                x['time_unit'] == time_unit]
         rhs = [x for x in json2['benchmarks'] if x['name'] == name and