[cleanup] Fix many deprecated v8 calls in extensions/renderer

Replaces many deprecated uses of v8 functions Object::Get/Set,
Value::ToString, in extensions/renderer with versions that return
Maybe/MaybeLocals or by using casts if there are already
checks that ensure the cast will always succeed.

Bug: v8:7279, v8:7283, v8:8562, chromium:913942
Change-Id: I4aeec76251bd11dfc274156d8d00586e04e9a99e
Reviewed-on: https://chromium-review.googlesource.com/c/1353985
Commit-Queue: Dan Elphick <[email protected]>
Reviewed-by: Devlin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#615543}
6 files changed