GTTF: Make EmbeddedTestServer always use its own thread for IO

GTTF: Make EmbeddedTestServer always use its own thread for IO

Otherwise we likely end up blocking the browser's IO thread,
especially with big responses that don't fit in the kernel
buffer. This blocks EmbeddedTestServer's IO thread,
and if it's also the browser's IO thread the browser can't
consume the response resulting in a hang.

BUG=310713
R=jam@chromium.org, rch@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=231075

[jam, 2013-10-23 23:24:14]

redirecting to rch who reviewed some of the earlier changes

[Ryan Hamilton, 2013-10-24 00:31:40]

net/ lgtm  (but I'm not a chrome/ or content/ owner)

[jam, 2013-10-24 00:41:39]

lgtm

[Paweł Hajdan Jr., 2013-10-25 19:17:33]

Committed patchset #4 manually as r231075 (presubmit successful).