Move ExtensionProcessManager to src/extensions, part 4

* Move c/b/extensions/extension_process_manager.h to
  extensions/browser/process_manager.h
* Rename ExtensionsProcessManager to ProcessManager
* Place it in the "extensions" namespace

BUG=313481
TEST=browser_tests, unit_tests
R=miket@chromium.org
TBR=sky@chromium.org for mechanical header file move affecting chrome/browser/

Review URL: https://codereview.chromium.org/62713003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233956 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233956