Make page_cycler.py fully measure memory for each page.

Telemetry makes the distinction between metrics collected for each page and for
each page set. The fine-grained memory metrics were collected only per page set
(although there was some histogram data collected per page).

This CL makes the memory metric module systematically report the whole data per
page (and not only per page set) so that we can measure pages independently.
The per page set memory metric collection is now a no-op.

BUG=323494

Review URL: https://codereview.chromium.org/91573003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239784 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239784