Often, tasks just need to call a single API method. By implementing
such tasks via a lambda, we save a lot of boilerplate. Additionally,
since lambdas are defined inside other function bodies, they have
access to private methods, which sometimes allows for better
encapsulation.

This CL introduces {CancelableLambdaTask} and
{CancelableIdleLambdaTask} and uses them to replace some custom tasks.
More can be refactored later.

R=ahaas@chromium.org

Bug: v8:8238
Change-Id: I88bd2c9bd57ebc32d082528f2e4251d741a0d021
Reviewed-on: https://chromium-review.googlesource.com/c/1256773
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56367}