ipc: Make MessageAttachment inherit from RefCountedThreadSafe.

Previously, it inherited from RefCounted, which was wrong, since it's possible
for multiple threads to simultaneously have scoped_ptr references to a single
MessageAttachment.

BUG=535711

Review URL: https://codereview.chromium.org/1408793008

Cr-Commit-Position: refs/heads/master@{#357216}
2 files changed