v8binding: Use explicitly-typed callback functions in custom elements

We're planning to not use cross origin ScriptStates everywhere,
especially in IDL callback function and callback interface.
Given that, we'd like to minimize use of ScriptState of callbacks,
which will be nullptr in case of cross origin, so that the call sites
of callbacks do not need to handle a SecurityError.

This patch is a preparation of https://crrev.com/c/1314023 and
removes the use of the ScriptState from custom elements.

Bug: 886588, 892755
Change-Id: I52068920b659efe119496ec092448cdc107e0631
Reviewed-on: https://chromium-review.googlesource.com/c/1325585
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606819}
23 files changed