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 <lushnikov@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556977}
11 files changed