[M108 Migration][UA] Refactor user agent strings

[M108 Migration][UA] Refactor user agent strings

This commit,
1. reduces system api access
2. moves combination of the user_agent_string to constructor
3. fixes script execution in gn file to get the chromium version
   in defined pattern
4. removes unnecessary code

These changes are required for User Agent String Test in Behavior TCT.

References:
https://review.tizen.org/gerrit/279991/
https://review.tizen.org/gerrit/281101/

Change-Id: I46787be994fdeada8e11d703a903b5069ed7de46
Signed-off-by: Chandan Padhi <c.padhi@samsung.com>