This CL removes the isolate_ property from the Cancelable class. The
isolate is actually not used in the class itself, only in sub-classes
which can store the isolate themselves.

R=jochen@chromium.org, mlippautz@chromium.org

Change-Id: If102fdea41b81fb3cb25782728f7fcbce9642815
Reviewed-on: https://chromium-review.googlesource.com/516704Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45535}