[tools/perf] Keep both browsers open from the start of dual browser test

At the start of the story set, open the browsers associated with all
stories. This makes sure that, when taking memory dumps, we always get
a dump for all browsers all the time.


Review-Url: https://codereview.chromium.org/2073713003
Cr-Commit-Position: refs/heads/master@{#400667}
1 file changed