Add IO Thread to TestBlinkWebUnitTestSupport

This is required to support moving code from content/renderer
to Blink (i.e., Onion Souping) that uses the IO Thread.

Bug: 928021
Change-Id: I67210ee7e958403d559c2fc99783699991e7ad92
Reviewed-on: https://chromium-review.googlesource.com/c/1450266
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628915}
2 files changed