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Android can have tabs without any web contents. Tab association logic
assumes that all tabs have web contents. This prevents Android from
cleanly doing session reassociation. This change adds a
SyncedTabDelegate which does not depend on existence of web contents.
It also ensures that SessionID for a tab does not change when web
contents get attached to the tab.

BUG=139670, 125549

Review URL: https://chromiumcodereview.appspot.com/15499005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@203071 0039d316-1c4b-4281-b951-d872f2087c98