[headless] Fix race in TargetDomainCreateTwoContexts test.

There's a race between completion of the load of the url provided to
Target.createTarget and the taking effect of Page.enable sent to the
created targets.

This patch resolves the race by separating target creation from page
loading to initiate the page load after Page.enable.

BUG=546953

Review-Url: https://codereview.chromium.org/2794883002
Cr-Commit-Position: refs/heads/master@{#461403}
1 file changed