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During login, GAIA sends the "Google-Accounts-SAML=Start" and
"Google-Accounts-SAML=End" headers to indicate when a redirect to a SAML
IdP starts and ends. This CL uses these headers to distinguish between
cookies set by GAIA and the IdP:
* Any cookies set between the start and the end of the redirect were
  created by the IdP.
* Any cookies set before the start or after the end of the redirect
  were created by GAIA.

This allows cookies set by the IdP to be copied to the user's session
while ensuring that GAIA cookies are not copied (these need to be
transferred to the session via GAIA's /MergeSession API).

BUG=381123
TEST=Extended browser test; also manual

Review URL: https://codereview.chromium.org/447013003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288068 0039d316-1c4b-4281-b951-d872f2087c98