[sdk] Support old backends that don't understand uniqueContextId.
[sdk] Support old backends that don't understand uniqueContextId.

With https://crrev.com/c/2602710 we broke support for Runtime.evaluate()
with different contexts for back-ends that don't yet support
uniqueContextId. This change restores support by using uniqueContextId
only when it was send by the back-end and otherwise using contextId as
before.

Fixed: chromium:1192621
Bug: v8:11268, chromium:1101897
Change-Id: I37909443eaea6fb4d92a1c258383a4753639c8b9
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2815125
Commit-Queue: Benedikt Meurer <024a1e0af501338d11c3b837c6193d11c140d792@chromium.org>
Auto-Submit: Benedikt Meurer <024a1e0af501338d11c3b837c6193d11c140d792@chromium.org>
Reviewed-by: Sigurd Schneider <f04d242e109628a27a0f75fd49a87722bcf291fd@chromium.org>
Reviewed-by: Andrey Kosyakov <03b08926e49d80331ab322c2b9e0332109fbfd39@chromium.org>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend