Enable out-of-process iframes benchmark using custom page set.

Enable out-of-process iframes benchmark using custom page set.

Since out-of-process iframes have different characteristics for memory
usage and page load times, we should have a separate basic page set to
have more control over performance measurements.

This CL adds such a page set and re-enables the benchmark. It also adds
another benchmark that executes without out-of-process iframes enabled
with the same new page set, which will allow us to compare regular Chrome
vs out-of-process enabled Chrome.

BUG=462323

Review URL: https://codereview.chromium.org/1250653002

Cr-Commit-Position: refs/heads/master@{#340592}