Show sync configuration UI in same tab as sign in flow when user refreshes sign in...

Show sync configuration UI in same tab as sign in flow when user refreshes sign in page.

The sign in flow is considered to be aborted when a refresh occurs.
That causes the source of the sign in to be cleared. Before this fix,
if a sign in was completed from a refreshed sign in page, the wrong
source was used to determine in which tab the sync configuration UI
should be displayed.

With this fix, the GAIA redirect URL is used to determine in which
tab the sync configuration UI should be displayed. This URL is maintained
by GAIA for the whole sign in flow even when refresh and/or back actions
occur (except for the "source" parameter). If the continue URL has no
"auto_close" parameter and is a blank page, the sign in tab must be
reused to show the sync UI. Otherwise, a new tab or an existing settings
tab must be used.

TEST=
1.Create a new User.>> On Sign-in Page>> Click on "Sign-in" without entering any Credentials.
2.Right Click >> Reload>> Continue.
3.Sign in with "Choose What To Sync" checked, with Proper Credentials.
4.Click on "Okay-got-it" in the Advanced Sync Interstitial Pop-up.
Expected result: Sync configuration UI is displayed in the same tab as the sign in flow.
This tab has focus and no new tab is created.
BUG=274196
TBR=rogerta@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23191003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@218736 0039d316-1c4b-4281-b951-d872f2087c98