rtc::ThreadManager::ThreadManager() calls rtc::InitCocoaMultiThreading()
on iOS so add base/maccocoathreadhelper.mm to the file to build on iOS.
Fixes the following linker error:
Undefined symbols for architecture x86_64:
"rtc::InitCocoaMultiThreading()", referenced from:
rtc::ThreadManager::ThreadManager() in librtc_base.a(thread.o)
BUG=459705
NOTRY=True
Review URL: https://codereview.webrtc.org/1810373003
Cr-Commit-Position: refs/heads/master@{#12085}
60624cd6bf