Allow the creation of partially spoofed user agents

Refactors user agent generation so that we can create partially spoofed user
agents for other platforms.

BUG=128570


Review URL: https://chromiumcodereview.appspot.com/10854251

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@153783 0039d316-1c4b-4281-b951-d872f2087c98
9 files changed