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This patch mostly consists of testing environment for web WebClient. While
WebClient intended to be base for a new DeviceProvider in DevTools frontend
(https://codereview.chromium.org/720133002/) it is convinient to run
tests on android. It lets to test the client directly against server
counterpart avoiding flakiness of network and cloud services.

TBR=jochen@chromium.org
BUG=383418

Review URL: https://codereview.chromium.org/736363002

Cr-Commit-Position: refs/heads/master@{#305131}