Remove duplicated code in offscreens's TryRestoreContextEvent

Now that we discard the resource provider for both real and synthetic
context losses, the only difference remaining is that for real context
losses, we need to wait for the backend to be up before trying the
restore.

Bug: 404257881
Change-Id: Idf16afbf07d40d3d9e11311a683052417a42dd60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6390056
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1441261}