extensions: Relocate pref name constants.

Move extension-related pref name constants used by the
ExtensionPrefs class from chrome/common/pref_names.h to
extensions/browser/pref_names.h. Remove "Extension" from the
beginning of the constant identifiers and "Pref" from the
end since it seemed redundant now that they're in the
extensions::pref_names namespace.

Also delete prefs::kExtensionBlacklistUpdateVersion since it
hasn't been used in about a year.

BUG=313284
TBR=isherman@chromium.org,sky@chromium.org

Review URL: https://codereview.chromium.org/140343002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245374 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245374