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This consists of:
1) Deletion of WillMeasurePage/DidMeasurePage and
page_specific_results_for_current_page (which does not
make sense since a results can be added any time between
StartTest/StopTest.
2) Moving trace_tag and current_page property to PageTestResults;
current_page is filled at StartTest and reset at StopTest.

The class itself will be deleted separately after all
of its subclasses are deleted or switched to OutputFormatter
(as per thread on telemetry@chromium.org).

BUG=383635

Review URL: https://codereview.chromium.org/403093002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285612 0039d316-1c4b-4281-b951-d872f2087c98