DevTools: implement Target.getBrowserContexts() method
DevTools: implement Target.getBrowserContexts() method

This patch implements Target.getBrowserContexts() method that
returns all the contexts created with Target.createBrowserContext()
calls.

R=dgozman, alexclarke, caseq

BUG: 836272
Change-Id: Ib39f947c5591ddb2d6158d8417bc1c8b82561d15
Reviewed-on: https://chromium-review.googlesource.com/1049266
Commit-Queue: Andrey Lushnikov <73889f071ff743ad62763f13ff61c52ca3ee19d0@chromium.org>
Reviewed-by: Dmitry Gozman <0ca14b32cbb359a30ac562af27dd84cc2813eacb@chromium.org>
Reviewed-by: Alex Clarke <9f4f272ef9b13bebfae45ae3fefff7bc1f90856a@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#556977}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7c173628db963ab1e459e5b857273fe41f2a81c2
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