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This patch adds tab sync performance tests, similar to the tests for the existing datatypes in live_sync/performance.  Changes to LiveSessionsSyncTest were required - most significantly the addition of AwaitSessionPropagation, which waits until a vector of urls can all be found in the model associator.  This type of wait loop previously existed as part of OpenTab, and worked with a single url.  AwaitSessionPropagation supports multiple urls, and is called from OpenTab, OpenMultipleTabs, and SessionsSyncPerfTest::UpdateTabs.

BUG=none
TEST=sync_performance_tests, sync_integration_tests

Review URL: http://codereview.chromium.org/7575026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@95675 0039d316-1c4b-4281-b951-d872f2087c98