Correctly compute script hashes

The hashes produced by ScriptDecoder, ScriptDecoderWithClient, and
DataPipeScriptDecoder don't match the hashes produced by
ParkableStringImpl::HashString, which could lead to incorrect behavior
in uncommon cases. This change makes the hashes consistent and adds a
debug check to prevent regressions.

Bug: 40244488, 40222504
Change-Id: Ifb770d764e8fff5183ed748e269e912085396c36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5902217
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1366833}