Remove ExecutionContextTask::taskNameForInstrumentation()

taskNameForInstrumentation() is a cause to define custom subclasses of
ExecutionContextTask and blocks removal of such custom subclasses.
This CL split it out to an argument of postTask() to enable further refactoring
of ExecutionContextTask subclasses.

BUG=478194

Review-Url: https://codereview.chromium.org/2098433002
Cr-Commit-Position: refs/heads/master@{#402079}
16 files changed