Move extension_misc::UnloadedExtensionReason from extension_constants.h to UnloadedExtensionInfo in extension.h.

This precipitated a IWYU fix for extension_constants.h.

BUG=298586,310174
TBR=koz@chromium.org,sky@chromium.org

Review URL: https://codereview.chromium.org/36023002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230377 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230377