Reduce v8::Local usage for templates in the generated bindings.

* `object_template` and `interface_template` are always
  `v8::ObjectTemplate`, so pass them through the whole stack as
  `v8::ObjectTemplate` instead of `v8::Template` and casting.
* `v8::Signature` is actually a cast of a `v8::FunctionTemplate`.
  `instance_template` is passed as a `v8::Template` and can be
  either a `v8::FunctionTemplate` or `v8::ObjectTemplate`, but
  in the cases where we cast `instance_template` to `v8::Signature`,
  it's guaranteed to be a `v8::FunctionTemplate`. Just do a direct
  cast from the `v8::Template` instead of first casting to
  `v8::FunctionTemplate`.
* Make conditional member installation use `CxxLikelyIfNode()`
  instead of `CxxUnlieklyIfNode()`. This makes the generator much
  more willing to declare the reusable `v8::Signature` outside of
  if() blocks, which leads to *many* fewer allocations in a few
  places (notably V8Window).

Fixed: 387392806
Change-Id: I8b330813f36913e12bfda7de68a3f1a28dde7b0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6153876
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1403317}