Issue #1783 - Part 2: Update ResizeObserver and resizeObserverSize.

This adds the later spec revision's contentBoxSize and borderBoxSize, and the
inlineSize and blockSize concepts.
The older spec contentRect remains functional as well for backwards
compatibility with the earlier spec (that was IMHO perfectly fine as it was...)

dom/bindings/Bindings.conf

@@ -736,6 +736,11 @@ DOMInterfaces = {
     'headerFile': 'mozilla/dom/ResizeObserver.h',
 },
 
+'ResizeObserverSize': {
+    'nativeType': 'mozilla::dom::ResizeObserverSize',
+    'headerFile': 'mozilla/dom/ResizeObserver.h',
+},
+
 'Response': {
     'binaryNames': { 'headers': 'headers_' },
 },