Component updater on-demand logic with Resource Throttle

This CL introduces the new way to ask for an on demand install when you have a network request that needs the component to be present:

ComponentUpdaterService::GetOnDemandResourceThrottle()

Returns a resource throttle and initiates a update check + install immediately. The network request associated with the request is unblocked once the component is installed.

The unittests are in progress here
https://codereview.chromium.org/55303002/

BUG=307193
TEST=none
R=darin@chromium.org, sorin@chromium.org, waffles@chromium.org

Review URL: https://codereview.chromium.org/25713007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232517 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232517
Undo priority mapping in preparation for the sync code change.

Feedback from our dogfood indicates that GUNS notifications should
normally go straight to the message center.  So, we will change the priority
mapping of GUNS notifications so that GUNS urgent messages map to chrome
medium priority.  This mapping will occur at the sync server.  So that the
sync server can do the mapping, I am undoing the special priority override
mapping that we had been doing on the client with this changelist.

This change must e checked in timed to go live shortly after the sync server
change goes live, I will work with the sync server dev to do that.

BUG=297255

Review URL: https://codereview.chromium.org/32593009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232518 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232518