Fix display.getInfo API crash issue on Linux.

Refer to[1], gdk_screen_get_monitor_plug_name() will return a NULL
pointer if the monitor name cannot be determined.

The crash is mainly caused by passing a NULL pointer to std::string
constructor. This patch is adding NULL check before construct std::string.

[1]:https://developer.gnome.org/gdk/unstable/GdkScreen.html#gdk-screen-get-monitor-plug-name

BUG=310289

Review URL: https://codereview.chromium.org/34523009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231007 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231007