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

Revert of [Android] Disable system_health.common_mobile on nexus 7 devices. (patchset #1 id:1 of https://codereview.chromium.org/2611753005/ )

Reason for revert:
Didn't fix.

Original issue's description:
> [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}
> Committed: https://chromium.googlesource.com/chromium/src/+/a73420c80a6038860cc35d1d6f1498c963ba2b52

TBR=nednguyen@google.com,sullivan@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=667470

Review-Url: https://codereview.chromium.org/2618013002
Cr-Commit-Position: refs/heads/master@{#441950}

tools/perf/benchmarks/system_health.py

@@ -77,10 +77,6 @@ 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'