Add a label in URLRequest NetLog to indicate the connection migration mode
it is running with.

QUIC Connection Migration: add a label in each URLRequest to indicate
connection migration mode it's running with so that URLRequest and associated
QuicSession can be differentiated in a single NetLog if multiple CronetEngines
with different experiment configurations are running at the same time.

Bug: 774622
Change-Id: I003f0ac83efbec7ac41d16a83ef5d0230d4aa7d1
Reviewed-on: https://chromium-review.googlesource.com/804296
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521210}
4 files changed