[Telemetry] Improve _GetProcJiffies operation with using grep.

tools/telemetry/telemetry/core/platform/linux_based_platform_backend_unittest.py

Index: tools/telemetry/telemetry/core/platform/linux_based_platform_backend_unittest.py
diff --git a/tools/telemetry/telemetry/core/platform/linux_based_platform_backend_unittest.py b/tools/telemetry/telemetry/core/platform/linux_based_platform_backend_unittest.py
index 6e706a52befb7780e6dfc1db80775c7785ed0817..04f786235b0a5d5c34eb4ad551981e68bb38d8db 100644
--- a/tools/telemetry/telemetry/core/platform/linux_based_platform_backend_unittest.py
+++ b/tools/telemetry/telemetry/core/platform/linux_based_platform_backend_unittest.py
@@ -8,6 +8,10 @@ import unittest
 from telemetry.core.platform import linux_based_platform_backend
 from telemetry.core import util
 
+util.AddDirToPythonPath(util.GetTelemetryDir(), 'third_party', 'mock')
+import mock  # pylint: disable=import-error
+
+
 
 class TestBackend(linux_based_platform_backend.LinuxBasedPlatformBackend):
 
@@ -48,10 +52,14 @@ class LinuxBasedPlatformBackendTest(unittest.TestCase):
       logging.warning('Test not supported')
       return
 
-    backend = TestBackend()
-    self.SetMockFileInBackend(backend, 'timer_list', '/proc/timer_list')
-    result = backend.GetCpuTimestamp()
-    self.assertEquals(result, {'TotalTime': 105054633.0})
+    with mock.patch.object(
+        linux_based_platform_backend.LinuxBasedPlatformBackend,
+        'RunCommand', return_value='jiffies: 10505463300') as mock_method:
+      backend = linux_based_platform_backend.LinuxBasedPlatformBackend()
+      result = backend.GetCpuTimestamp()
+      self.assertEquals(result, {'TotalTime': 105054633.0})
+    mock_method.assert_call_once_with(
+        ['grep', '-m', '1', 'jiffies:','/proc/timer_list'])
 
   def testGetMemoryStatsBasic(self):
     if not linux_based_platform_backend.resource: