Reland of Migrate DeviceUtils.ReadFile to adb_wrapper

tools/telemetry/telemetry/core/backends/chrome/android_browser_backend.py

Index: tools/telemetry/telemetry/core/backends/chrome/android_browser_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/android_browser_backend.py b/tools/telemetry/telemetry/core/backends/chrome/android_browser_backend.py
index 4b486beab404c4e0f3bb6dddd9156522fa8676ac..73087ea76715633dafc7c86dfebb8702115306d1 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/android_browser_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/android_browser_backend.py
@@ -223,7 +223,8 @@ class AndroidBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
       # be a  Telemetry created one. This is to prevent a common bug where
       # --host-resolver-rules borks people's browsers if something goes wrong
       # with Telemetry.
-      self._saved_cmdline = ''.join(self._adb.device().ReadFile(cmdline_file))
+      self._saved_cmdline = self._adb.device().ReadFile(
+          cmdline_file, as_root=True)

[perezju, 2014/12/10 17:22:27]

Turns out, this was silently failing (and returning empty contents) because of
the missing as_root=True

[jbudorick, 2014/12/11 01:56:24]

... and implementing ReadFile with RunShellCommand(..., check_return=True)
exposed that error?

       if '--host-resolver-rules' in self._saved_cmdline:
         self._saved_cmdline = ''
       self._adb.device().WriteFile(cmdline_file, content, as_root=True)