Since the AST graph builder is gone, no variable should be named osr_ast_id.
This CL replaces it with osr_offset. It designates the offset of the bytecode
where the OSRing was triggered.

Bug: 
Change-Id: Ia53a83b09f917fcd0174da685a18edd3ee3aa01f
Reviewed-on: https://chromium-review.googlesource.com/621008Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47566}