Expect that IntersectionObserver API may not be available

Related issue:
- https://github.com/uBlockOrigin/uMatrix-issues/issues/235

The popup-resize functionality won't be available if the
IntersectionObserver API is not available. The purpose
of the popup-resize code is to adjust the matrix visuals
on small display -- i.e. on mobile or when the popup panel
is used in the overflow menu in Firefox.

src/js/popup.js

@@ -77,6 +77,11 @@ const resizePopup = (( ) => {
 
     // The purpose of `xobserver` is to initiate the resize handler only
     // when the popup panel is actually visible.
+
+    // https://github.com/uBlockOrigin/uMatrix-issues/issues/235
+    if ( self.IntersectionObserver instanceof Object === false ) {
+        return ( ) => { };
+    }
     let xobserver = new IntersectionObserver(intersections => {
         if ( intersections.length === 0 ) { return; }
         if ( intersections[0].isIntersecting === false ) { return; }