[Cronet] Add integrated mode support for Cronet

In integrated mode, CronetEngine is allowed to use the shared network
task runner by other Chromium-based clients (like webview, Chrome
Android, Cronet) without self-initialization, which make it avaialbe to
expose Cronet Java API in Chromium-based clients without creating 2
different network stacks.

Besides, multiple CronetEngine instances could also share the same
network stack in integrated mode.

Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester
Change-Id: I83678f78f420d033db2a0df75a9934abf4021465
Reviewed-on: https://chromium-review.googlesource.com/1136338
Commit-Queue: Shenghuai Ji <[email protected]>
Reviewed-by: Misha Efimov <[email protected]>
Cr-Commit-Position: refs/heads/master@{#578149}
8 files changed