Don't rely on untrusted events in site isolation browser test.

The site isolation browser test for two iframes showing selects
simultaneously relies on untrusted events performing the default
action.

Since untrusted event dispatching will be removed from blink soon
fix this test.

BUG=520519

Review-Url: https://codereview.chromium.org/2077683003
Cr-Commit-Position: refs/heads/master@{#400397}
2 files changed