Remove Increment/DecrementKeepAliveRefCount from RPH content public API

These methods were not actually used in any tests, and were cargo-
culted over to MockRenderProcessHost. Remove these and clean up the
content public API.

Test: No behaviour change
Bug: 323753235
Change-Id: If6b5208429940f2b88fce302f2c352e04d723532
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5422132


Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282762}