Run upload tasks as USER_INITIATED in drive::SyncClient at first.

See the associated bug for the context. In short, the first UPLOAD
tasks from operations are always pushed as a result of explicit
user operation on Drive files and thus should be run immediately.

BUG=315000

Review URL: https://codereview.chromium.org/63153004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237046 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237046