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(1) Add a boolean |restrict_to_one_preconnect_for_proxies| to network
session params. This allows us to run multiple experiments using a single
field trial. Next CL will add the variation param
|race_preconnects_to_proxies| to experiment with racing the alternative
and main jobs for proxy preconnects.

(2) Remove an extra parameter in
HttpStreamFactoryImpl::Job::Delegate::OnStreamReady() method.

BUG=667471,671291
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

Review-Url: https://codereview.chromium.org/2600943002
Cr-Commit-Position: refs/heads/master@{#440893}