Previously ToNumber could be called with an empty context. In a previous
CL (https://crrev.com/c/v8/v8/+/2078580) we added DCHECKS to make sure
that some paths were not using the empty context. Now we are doing the
next step of adding a primitive to separate the cases.

Small update from delphick@ to get the builtin descriptor right.

Bug: v8:6949, v8:10933
Change-Id: Ie40b169f680f60a08eb26fac1fcfcef7d6169e65
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2428863
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70162}