The `SaveContext` operation in `AsyncCompileJob::CompileTask` allocates
a handle. However, the platform implementation may not be able
to provide a `HandleScope`, since it cannot tell whether the isolate
is disposed (and the task canceled) at the time it runs the task;
so it is an API requirement of `CancelableTask` is that `RunInternal()`
does not leak any handles into outside scopes.

Change-Id: I86db36ddc71f774a31d5bc13b7399ef961374d6f
Reviewed-on: https://chromium-review.googlesource.com/668397Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48042}