Add end-to-end browser tests for PluginVm setup flow

Adds end-to-end setup flow browser tests.
This CL also adds DCHECKs that check that UI related calls made in the
UI thread.

Test: browser_tests --gtest_filter="PluginVm*"
Bug: 920988
Change-Id: Ia3ae225f44f05cde4468b72f49a7ecbaa250c42c
Reviewed-on: https://chromium-review.googlesource.com/c/1494881
Commit-Queue: Olya Kalitova <okalitova@chromium.org>
Reviewed-by: Igor <igorcov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637253}
3 files changed