Throughput: Change the min number of requests in flight required

In network quality estimator (NQE), change the minimum number of
requests in flight required before a throughput observation can be
taken. The parameter is controlled using field trial params, with
default value set to 1.

BUG=725146

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

Review-Url: https://codereview.chromium.org/2893933004
Cr-Commit-Position: refs/heads/master@{#473755}
6 files changed