Make multiple-connections.html less aggressive.

Make multiple-connections.html less aggressive.

The test WebSocketBrowserTest.SSLConnectionLimit is flaky because it
attempts to connect 30 WebSockets all at once. This triggers the
per-renderer WebSocket connection throttling, and sometime that makes it
take over 5 seconds.

Make similar modifications as in https://crrev.com/1021143002 to only
attempt 2 connections at a time. This avoids triggering the throttling.

BUG=481667
TEST=browser_tests WebSocketBrowserTest.SSLConnectionLimit

Review URL: https://codereview.chromium.org/1153533003

Cr-Commit-Position: refs/heads/master@{#331553}