[ios blink] Implement compound backing passthrough accessors

We now use the compound image backing path for shared memory GMBs on ios
blink so it's important that ProduceOverlay work as expected. This CL
also fixes the other virtual accessors so a similar issue doesn't arise
if we ever use this backing on other platforms.

Change-Id: I127723b9b4a7c2d121084a1f5a3fefb43ec57b9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6490729
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Auto-Submit: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1451944}