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Currently, browser contexts created with `Target.createBrowserContext()`
method will die once the protocol session is closed. This is inconvenient
and inconsistent with headless behavior.

This patch introduces a new DevToolsBrowserContextManager class to take
care of browser context management. The instance of this class is shared
between all protocol sessions.

R=dgozman
BUG=836272

Change-Id: If8b4b542d1a861298ef417a40b44c218d04d2f22
Reviewed-on: https://chromium-review.googlesource.com/1049110
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556923}