At the 2013 WebKit Contributors Meeting this year one of the discussions was upstreaming iOS WebKit:
Discussion: What do we do about ASSERT(isMainThread())?
DECISION: Leave it named isMainThread()…
On iOS isMainThread() means that the current thread is either the MainThread or WebThread and the current thread has the WebThreadLock.
Created attachment 208463 [details]
[PATCH] Proposed Fix
Attachment 208463 [details] did not pass style-queue:
Failed to run "['Tools/Scripts/check-webkit-style', '--diff-files', u'Source/WTF/ChangeLog', u'Source/WTF/wtf/ios/WebCoreThread.cpp', u'Source/WTF/wtf/ios/WebCoreThread.h', u'Source/WTF/wtf/mac/MainThreadMac.mm']" exit_code: 1
Source/WTF/wtf/ios/WebCoreThread.h:28: #ifndef header guard has wrong style, please use: WTF_WebCoreThread_h [build/header_guard] 
Total errors found: 1 in 4 files
If any of these errors are false positives, please file a bug against check-webkit-style.