[Android] Disable system_health.common_mobile on nexus 7 devices.

BUG=667470
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.perf:linux_perf_cq;master.tryserver.chromium.perf:winx64_10_perf_cq

Review-Url: https://codereview.chromium.org/2611753005
Cr-Commit-Position: refs/heads/master@{#441690}

tools/perf/benchmarks/system_health.py

@@ -77,6 +77,10 @@ class MobileCommonSystemHealth(_CommonSystemHealthBenchmark):
         possible_browser.platform.GetDeviceTypeName() == 'Nexus 5X'):
       return True
 
+    # http://crbug.com/667470
+    if 'Nexus 7' in possible_browser.platform.GetDeviceTypeName():
+      return True
+
     return possible_browser.platform.GetDeviceTypeName() == 'Desktop'