GitLab

Implements and tests the Context child process spawning aspects of
ContextProvider.

Additionally moves non-main portions of "//webrunner:webrunner" into a
component, so that they can be called from unit tests.

Bug: 850743
Change-Id: I4f4aafac71ac56d72c6931db3751201bc516d92a
Reviewed-on: https://chromium-review.googlesource.com/1096453
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568161}