Add nullable types to js type mapping

This eliminates the usage of EnsureNullable, which may allow us to get
rid of this helper method entirely.

Change-Id: I94e6003c56337f15ee8f4cc07e5e2b33c10ac2eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5466235


Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Fred Shih <ffred@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1289605}