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We add the list of tab's markers to traces as metadata and later reconstruct the maps from tab markers to renderer thread in trace_event_importer. This makes it possible to reconstruct the timeline model with the mapping between tabs & renderer threads from the raw json tracing data. 

BUG=

Review URL: https://codereview.chromium.org/309533016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275671 0039d316-1c4b-4281-b951-d872f2087c98