Move JobPriority from parameter to scheduler state

With future introduction of access token request not all calls to
TrySyncSessionJob will result in posted tasks. Still fact that
TrySyncSessionJob was called with CANARY_PRIORITY needs to be preserved.

I'm making next job priority part of scheduler's state. TryCanaryJob
sets it to canary, any call to TrySyncSessionJobImpl resets it back to
normal.

BUG=259913

Review URL: https://codereview.chromium.org/124423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242997 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242997