Save and restore a preexisting command line on exit in android run-webkit-tests

The command line content_shell uses on Android is determined by the contents of /data/local/tmp/content-shell-command-line.

Currently run-webkit-tests overwrites that file repeatedly, and does not restore the original contents on exit (or delete the file if it didn't previously exist). This made switching back and forth between running the layout tests and running content_shell interactively more painful than it need be.


R=peter@chromium.org
BUG=303390

Review URL: https://codereview.chromium.org/27627002

git-svn-id: svn://svn.chromium.org/blink/trunk@159895 bbb929c8-8fbe-4397-9dbb-9b2b20218538