At the moment the inspector tests do not dispose the isolate. This is a
problem because the disposal of the isolate is used to stop the
execution of background tasks. The missing disposal of the isolate
caused flaky tests on the bots recently. With this CL the isolates of
the inspector tests get disposed.

The disposal of the isolate requires the following changes: 1) Store the
isolate in a unique_ptr so that it gets disposed when the isolate-data
gets disposed. It is necessary to use the unique_ptr so that the isolate
gets disposed after other members of isolate-data get disposed.  2)
Dispose all sessions. The reason is that the sessions require the
isolate to exist when they get disposed because they own handles.
Sessions, however, are stored in a static map, whereas the isolate is
stored indirectly in a local variable of the main function. Since local
variables get disposed before the static map is cleared, we have to
clear the map before the end of the main function.

R=kozyatinskiy@chromium.org

Change-Id: Icb33184de254638b6cdfb899e940f18e6064cd69
Reviewed-on: https://chromium-review.googlesource.com/774885Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49424}