[ATP] Support gin bindings for Automation.

ATP will re-implement the V8 bindings for Automation and other
extension APIs used by accessibility feature component extensions.
This change makes it easier to re-use existing bindings code:
*Allows ATP to use gin::CreateFunctionTemplate while extensions
continues to use v8::FunctionTemplate::New
*Requires a change to add a way to get the original
V8::FunctionCallbackInfo from a gin::Arguments.

If we don't add the getter to gin::Arguments, all bindings code
across all extensions used by Accessibility Features would need
to be updated to take a gin::Bindings, or we'd need to build
a custom way outside of gin::Arguments to get the
FunctionCallbackInfo.

For diagrams and high-level overview of the context of this
change see go/chromeos-atp-v8-design (WIP).

Bug: 1357889
AX-Relnotes: N/A
Test: Existing
Change-Id: If4552a4c08ae8a32a2c0ee9288891dcd8c8a64a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3985889
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1067695}