Adding task runner/ message loop to tests that use IsolateHolder.

For adding v8 heap memory dump provider, the gin::IsolateHolder needs
to have a task runner at initialization. The tests which don't
initiallize task runner when initialization are changed in this CL to
have either the message loop initializaed earlier or a dummy task
runner. Refer crrev.com/1088683003 for the dump provider.

BUG=476013

Review URL: https://codereview.chromium.org/1130433002

Cr-Commit-Position: refs/heads/master@{#328531}
5 files changed