chromeos: More power-related cleanup.

This consolidates SuspendObserver and ResumeObserver in a
PowerEventObserver class within ash/system/chromeos/power
and moves extension-related code into an
ExtensionSystemEventObserver class.

BUG=none

Review URL: https://codereview.chromium.org/26692002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227855 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227855
Forget opener and groups on tab move.

This mirrors the behavior on tab detach. We probably shouldn't behave
differently if you move a tab only horizontally and if you briefly detach it in
getting it to its final location.

BUG=298180
TEST=TabStripModelTest.MoveSelectedTabsTo_ForgetGroups

Review URL: https://codereview.chromium.org/26261006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227856 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227856