Reland of Split PendingScript into PendingScript and ClassicPendingScript

This is preparation for introducing ModulePendingScript.

This CL shouldn't change the behavior.

This has been reverted due to crashing (Issue 711703) because
ResourceOwner's prefinalizer is called before PendingScript's prefinalizer,
causing CheckState() assertion failure.
This reland fixes this issue by registering PendingScript::Dispose() also
as the prefinalizer of ClassicPendingScript, which is called before
ResourceOwner's prefinalizer.

A unit test for the crash will be added by
https://codereview.chromium.org/2828973002/.

BUG=594639, 686281, 711703

Review-Url: https://codereview.chromium.org/2653923008
Cr-Original-Commit-Position: refs/heads/master@{#464494}
Committed: https://chromium.googlesource.com/chromium/src/+/d512803299c01324b92ffd8c962da97e3bc8ccff
Review-Url: https://codereview.chromium.org/2653923008
Cr-Commit-Position: refs/heads/master@{#466899}
10 files changed