Due to quirks of the Cocoa runtime several platform-specific fixes were
in place. See the deleted files maccocoathreadhelper and
scoped_autorelease_pool for examples. There is no way to do a stack-based
RAII autoreleasepool that is compatible with ARC, and autoreleasepool
blocks can't be used with c++. The solution was to separate out the
implementation of some methods in thread.h to an ObjC++ file for Darwin
platforms, allowing us to get rid of the helper classes and enable ARC
everywhere.
BUG=webrtc:6412
Review-Url: https://codereview.webrtc.org/2784483002
Cr-Commit-Position: refs/heads/master@{#17436}
