Bug 1756750 - Ensure we observe SVG glyph changes inside <svg:text>. r=jfkthame

This gets done usually in BreakSink::Finish, but we don't do
line-breaking in SVG Text so we need to do this here instead.

Do you know where I could crib a test for this?

Depends on D139964

Differential Revision: https://phabricator.services.mozilla.com/D139965

layout/generic/nsTextFrame.cpp

@@ -1753,6 +1753,9 @@ void BuildTextRunsScanner::FlushFrames(bool aFlushLineBreaks,
 
   if (aFlushLineBreaks) {
     FlushLineBreaks(aSuppressTrailingBreak ? nullptr : textRun.get());
+    if (!mDoLineBreaking) {
+      CreateObserversForAnimatedGlyphs(textRun.get());
+    }
   }
 
   mCanStopOnThisLine = true;