Reland of Migrate DeviceUtils.ReadFile to adb_wrapper (try 3)

build/android/pylib/instrumentation/test_runner.py

Index: build/android/pylib/instrumentation/test_runner.py
diff --git a/build/android/pylib/instrumentation/test_runner.py b/build/android/pylib/instrumentation/test_runner.py
index fe444e91eff736124525fbbf34ea53f6d69cd2c6..888efcbeda09920f84b85867fb8fa9cfe5d5e7e3 100644
--- a/build/android/pylib/instrumentation/test_runner.py
+++ b/build/android/pylib/instrumentation/test_runner.py
@@ -235,10 +235,8 @@ class TestRunner(base_test_runner.BaseTestRunner):
           '/data/data/com.google.android.apps.chrome/files/PerfTestData.txt',
           as_root=True)
 
-      if json_string:
-        json_string = '\n'.join(json_string)
-      else:
-        raise Exception('Perf file does not exist or is empty')
+      if not json_string:
+        raise Exception('Perf file is empty')
 
       if self.options.save_perf_json:
         json_local_file = '/tmp/chromium-android-perf-json-' + raw_test_name