[Telemetry] Fix page_cycler histogram output.
[Telemetry] Fix page_cycler histogram output.

1. The existing page cycler output uses V8.Memory... instead of V8_Memory... and
   we want the traces to stay the same when we switch to telemetry for the
   page cyclers, so remove the bit that replaces '.' with '_' in histogram
    names.
2. Make the page_cycler benchmark use the histogram_measurement module. This
   fixes a bug where the test didn't work against reference builds because
   getHistogram isn't defined there.
3. Do print average histogram data if there is only 1 URL. In the case of 1 URL
   the _by_url output is suppressed, so it is necessary to print it.

BUG=196411
TEST=tools/perf/run_multipage_benchmarks --browser=system page_cycler tools/perf/page_sets/page_cycler/moz.json

Review URL: https://codereview.chromium.org/12580007

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@188766 0039d316-1c4b-4281-b951-d872f2087c98

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