Bug 1696721 part 1 - Fix incorrect span sizing on ruby with border / padding. r=dholbert

This issue is revealed by the test for this bug in a later part, so it's also covered by that test.

Differential Revision: https://phabricator.services.mozilla.com/D109226

layout/generic/nsRubyFrame.cpp

@@ -129,10 +129,9 @@ void nsRubyFrame::Reflow(nsPresContext* aPresContext,
   }
   NS_ASSERTION(aReflowInput.AvailableISize() != NS_UNCONSTRAINEDSIZE,
                "should no longer use available widths");
-  nscoord availableISize = aReflowInput.AvailableISize();
-  availableISize -= startEdge + borderPadding.IEnd(frameWM);
-  aReflowInput.mLineLayout->BeginSpan(this, &aReflowInput, startEdge,
-                                      availableISize, &mBaseline);
+  nscoord endEdge = aReflowInput.AvailableISize() - borderPadding.IEnd(frameWM);
+  aReflowInput.mLineLayout->BeginSpan(this, &aReflowInput, startEdge, endEdge,
+                                      &mBaseline);
 
   for (RubySegmentEnumerator e(this); !e.AtEnd(); e.Next()) {
     ReflowSegment(aPresContext, aReflowInput, aDesiredSize.BlockStartAscent(),