[Telemetry] Fix running certain benchmarks on user builds

showmap may not exist. For example, see the Moto E:
http://chromegw/i/chromium.perf/builders/Android%20MotoE%20Perf/builds/494/steps/page_cycler.intl_hi_ru/logs/stdio

This causes us to degrade gracefully and not output memory metrics.

BUG=

Review URL: https://codereview.chromium.org/431243002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287166 0039d316-1c4b-4281-b951-d872f2087c98

tools/telemetry/telemetry/core/platform/android_platform_backend.py

@@ -138,6 +138,8 @@ class AndroidPlatformBackend(
 
   def GetMemoryStats(self, pid):
     memory_usage = self._device.GetMemoryUsageForPid(pid)
+    if not memory_usage:
+      return {}
     return {'ProportionalSetSize': memory_usage['Pss'] * 1024,
             'SharedDirty': memory_usage['Shared_Dirty'] * 1024,
             'PrivateDirty': memory_usage['Private_Dirty'] * 1024,