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This adds a Builder class for UrlRequestContextConfig, so that the rather
large constructor can be avoided when many of the arguments should remain
as their defaults.
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

Review-Url: https://codereview.chromium.org/2567093002
Cr-Commit-Position: refs/heads/master@{#442902}