Switch guest profile from forced-incognito to UI mods like ChromeOS.

There have been issues with the way forced-incognito causes some things to behave and various menu items to appear.  We now follow the ChromeOS model (merging several instances of conditional ChromeOS code) of simply using the incognito profile when opening the window.

This change should fix some of the issues being seen in the field, simplify Guest profiles going forward, bring Desktop and ChromeOS experience closer together, and generally reduce Technical Debt from special-case scenarios.

BUG=103846

Review URL: https://codereview.chromium.org/164333007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251803 0039d316-1c4b-4281-b951-d872f2087c98
Have chromedriver use chrome/common/chrome_constants.cc directly rather than having to hardcode a copy or depend on the full target itself.

akin to https://codereview.chromium.org/163763004/

BUG=343692

Review URL: https://codereview.chromium.org/165053002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251804 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251804