[Extensions] Add CheckSoon() method to ExtensionUpdater

Change ExtensionService::CheckForUpdates() to use CheckSoon().

Change sync code to not worry about calling CheckForUpdates() at most
once.

BUG=
TEST=

Review URL: http://codereview.chromium.org/6832001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@81501 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed