Remove base::RefCountedSharedMemory (unused)

It is wrapping a deprecated class base::SharedMemory. All users of
base::RefCountedSharedMemory have been updated and this class is no
more used, it can be removed.

Chromium Shared Memory Refactor design doc:
https://docs.google.com/document/d/1lk2-7W7Cy7FDqG-6lgS8LRkzXABYTOytf6VUsWNf8-0/edit#heading=h.7nki9mck5t64

Bug: 795291
Change-Id: I67a7fa8a6103fc0813171c0a0b5b2965809cef2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566294
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651177}
13 files changed