Telemetry multipage testing fw: Enable running multiple benchmarks in the same run.

Telemetry multipage testing fw: Enable running multiple benchmarks in the same run.

This way we can record a data set which supports many benchmarks.

BUG=158323

[marja, 2012-11-12 11:58:17]

ptal

[nduca, 2012-11-12 20:06:04]

recapping from an offline chat:

I think the way to do this is to have a totally different thing from the
multi_page_benchmark.py that is called telemetry.page_set_recorder or something.

if we get this right, then you dont have to run any benchmarks --- you just have
to run all interactions on every page

so you'd have ./tools/perf/update_recordings.py <pageset> that would ask
telemetry to call page_set_recorder.

it would run all interactions on all pages and save out the wpr. And then we'd
remove the --record option entirely.