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The https://crrev.com/555640 regressed headless implementation of
Target.disposeBrowserContext() method and omitted the actual context
closing.

This patch brings back the missing method.

R=dgozman, alexclarke
TBR=alexclarke

Change-Id: I28677381be9fbb86daa5445978e9c0ce4bf388f7
Reviewed-on: https://chromium-review.googlesource.com/1050983
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557087}