Issue #1641 - Implement CSS flow-root keyword

This is just a clean port of 1322191 and follow-up 1325970. It really seems to add create a new way to access existing code relating to block formatting and floating elements rather than implementing new functionality, and it is mercifully straightforwards.

layout/generic/ReflowInput.cpp

@@ -844,6 +844,7 @@ ReflowInput::InitFrameType(nsIAtom* aFrameType)
     case StyleDisplay::Flex:
     case StyleDisplay::WebkitBox:
     case StyleDisplay::Grid:
+    case StyleDisplay::FlowRoot:
     case StyleDisplay::RubyTextContainer:
       frameType = NS_CSS_FRAME_TYPE_BLOCK;
       break;