[Telemetry] Allow page cycler to run on devices that don't report CommitCharge.

BUG=400734

Review URL: https://codereview.chromium.org/440323002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287756 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/metrics/memory.py

@@ -59,7 +59,10 @@ class MemoryMetric(Metric):
   def __init__(self, browser):
     super(MemoryMetric, self).__init__()
     self._browser = browser
-    self._start_commit_charge = self._browser.memory_stats['SystemCommitCharge']
+    start_memory_stats = self._browser.memory_stats
+    self._start_commit_charge = None
+    if 'SystemCommitCharge' in start_memory_stats:
+      self._start_commit_charge = start_memory_stats['SystemCommitCharge']
     self._memory_stats = None
     self._histogram_start = dict()
     self._histogram_delta = dict()
@@ -125,6 +128,7 @@ class MemoryMetric(Metric):
     AddResultsForProcesses(results, self._memory_stats,
                            metric_trace_name=trace_name)
 
+    if self._start_commit_charge:
       end_commit_charge = self._memory_stats['SystemCommitCharge']
       commit_charge_difference = end_commit_charge - self._start_commit_charge
       results.AddValue(scalar.ScalarValue(