Move c/c/e/extension_set to top-level extensions/

Move extension_set.h/cc and extension_set_unittest to extensions/common/.
Remove unused includes.
Add includes where transitive includes were used.
Update DEPS.
No functional changes.

BUG=162530
TBR=sky@chromium.org (c/b/autocomplete, c/b/ui, c/test)
TBR=jam@chromium.org (c/b/automation)
TBR=atwilson@chromium.org (c/b/background, c/b/sync, c/b/policy)
TBR=bauerb@chromium.org (c/b/browsing_data, c/b/content_settings, c/b/managed_mode)
TBR=satorux@chromium.org (c/b/chromeos)
TBR=yurys@chromium.org (c/b/devtools)
TBR=thestig@chromium.org (c/b/media_galleries)
TBR=stevenjb@chromium.org (c/b/notifications)
TBR=battre@chromium.org (c/b/profile_resetter)
TBR=dmazzoni@chromium.org (c/b/speech)
TBR=kinuko@chromium.org (c/b/sync_file_system)
TBR=pkotwicz@chromium.org (c/b/themes)

Review URL: https://codereview.chromium.org/102103005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241131 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241131