Oilpan library: Various user code fixes /2

Fixes
- forward declarations;
- includes
- IsDeleted check for Member

Bug: 1056170
Change-Id: Ifc24200dec44aecb027e4d1ccf54ca9ed4ac63e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2656041


Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#848169}