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

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..94390bbe9c7659fdf095cb2da04d2fa2dd3b6cd4 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/android_browser_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/android_browser_backend.py
@@ -153,7 +153,7 @@ class AndroidBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
 
     # Initialize fields so that an explosion during init doesn't break in Close.
     self._backend_settings = backend_settings
-    self._saved_cmdline = ''
+    self._saved_cmdline = None
     self._target_arch = target_arch
     self._saved_sslflag = ''
 
@@ -219,24 +219,33 @@ class AndroidBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
         self._adb.IsUserBuild())
 
     try:
-      # Save the current command line to restore later, except if it appears to
-      # 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))
+      # Save the current command line to restore later
+      self._saved_cmdline = self._adb.device().ReadFile(
+          cmdline_file, as_root=True)
+      # ... except if it appears to be a  Telemetry created one. This is to
+      # prevent common bug where --host-resolver-rules borks people's browsers
+      # if something goes wrong with Telemetry.
       if '--host-resolver-rules' in self._saved_cmdline:
-        self._saved_cmdline = ''
+        self._saved_cmdline = None
+    except device_errors.CommandFailedError: # file may not exist
+      self._saved_cmdline = None
+
+    try:
       self._adb.device().WriteFile(cmdline_file, content, as_root=True)
-    except device_errors.CommandFailedError:
-      logging.critical('Cannot set Chrome command line. '
-                       'Fix this by flashing to a userdebug build.')
+    except device_errors.CommandFailedError as exc:
+      logging.critical('Cannot set Chrome command line: %s', str(exc))
+      logging.critical('May be fixed by flashing to a userdebug build.')
       sys.exit(1)
 
   def _RestoreCommandLine(self):
     cmdline_file = self._backend_settings.GetCommandLineFile(
         self._adb.IsUserBuild())
-    self._adb.device().WriteFile(cmdline_file, self._saved_cmdline,
-                                 as_root=True)
+    if self._saved_cmdline is not None:
+      self._adb.device().WriteFile(cmdline_file, self._saved_cmdline,
+                                   as_root=True)
+    else:
+      self._adb.device().RunShellCommand(['rm', cmdline_file],
+                                         as_root=True)
 
   def Start(self):
     self._SetUpCommandLine()