Clean up differences between google3 and Chromium.

This involves introducing a bunch of new platform interfaces, such as:
* QuicFakeEpollServer
* QuicRunSystemEventLoopIteration
* QUIC_TEST_DISABLED_IN_CHROME
* QuicGetTestMemoryCachePath

R=rch@chromium.org

Change-Id: Iaafc71908337905a238ae0c7a4675c00f7676863
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504455
Commit-Queue: Victor Vasiliev <vasilvv@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637974}
39 files changed