Convert ExtensionProcessManager to BrowserContext, part 1

Part of moving ExtensionProcessManager to src/extensions.
* Introduce more test coverage
* Convert most usage of Profile to BrowserContext
* Separate out "master" vs. incognito profiles/contexts
* Move switches into extensions/common/switches.h

BUG=313481
TEST=unit_tests ExtensionProcessManager* and browser_tests ExtensionProcessManager*
R=miket@chromium.org

Review URL: https://codereview.chromium.org/52983004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232456 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232456