Document skia::textlayout::Run is non-assignable.

This class has a const member and so is non-assignable. Make it obvious
to the reader that this is a property to be preserved instead of just an
accident.

Change-Id: If269f3aea95b98a8d5c05971af53d222a2525f2f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295055
Auto-Submit: Ben Wagner <bungeman@google.com>
Reviewed-by: Julia Lavrova <jlavrova@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

modules/skparagraph/src/Run.h

@@ -64,7 +64,9 @@ public:
         size_t index,
         SkScalar shiftX);
     Run(const Run&) = default;
+    Run& operator=(const Run&) = delete;
     Run(Run&&) = default;
+    Run& operator=(Run&&) = delete;
     ~Run() = default;
 
     void setMaster(ParagraphImpl* master) { fMaster = master; }