Correct net::EmbeddedTestServer usage in frame_host tests.

Adding request handlers to the EmbeddedTestServer after it has started
isn't thread safe.

This CL updates some tests to only initialize the server before adding
the CrossSiteRedirector handler so that it can be done in a thread
safe manner. The server's IO thread is started once the handler is
added.

BUG=546060
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2491073002
Cr-Commit-Position: refs/heads/master@{#431538}
6 files changed