Add some sanity checks inside WebBackForwardList::addItem() to help debug <rdar://problem/46399319>. This crash seems to indicate that WebBackForwardList::m_entries can contain duplicate entries somehow.
Created attachment 356611 [details]
Attachment 356611 [details] did not pass style-queue:
ERROR: Source/WebKit/UIProcess/WebBackForwardList.cpp:95: More than one command on the same line [whitespace/newline] 
ERROR: Source/WebKit/UIProcess/WebBackForwardList.cpp:164: More than one command on the same line [whitespace/newline] 
Total errors found: 2 in 2 files
If any of these errors are false positives, please file a bug against check-webkit-style.