[Telemetry] Don't restore command lines that will break Chrome.

A common problem is that if something goes wrong with Telemetry, the command
line file which points all network traffic at Web Page Replay can be left in
place which results in users not understanding why no pages will load.

So if we detect this setting, simply clear the file.

BUG=

Review URL: https://codereview.chromium.org/108573008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242109 0039d316-1c4b-4281-b951-d872f2087c98

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

@@ -189,7 +189,7 @@ class AndroidBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
     # Initialize fields so that an explosion during init doesn't break in Close.
     self._adb = backend_settings.adb
     self._backend_settings = backend_settings
-    self._saved_cmdline = None
+    self._saved_cmdline = ''
     self._port = adb_commands.AllocateTestServerPort()
 
     # Kill old browser.
@@ -296,7 +296,13 @@ class AndroidBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
       self.Close()
       raise
     finally:
-      self._SetCommandLineFile(self._saved_cmdline or '')
+      # Restore the saved command line if it appears to have come from a user.
+      # If it appears to be a Telemetry created command line, then don't restore
+      # it. This is to prevent a common bug where --host-resolver-rules borks
+      # people's browsers if something goes wrong with Telemetry.
+      self._SetCommandLineFile(
+          self._saved_cmdline
+          if '--host-resolver-rules' not in self._saved_cmdline else '')
 
   def GetBrowserStartupArgs(self):
     args = super(AndroidBrowserBackend, self).GetBrowserStartupArgs()