Update Slide::getName to return a const reference.

The Viewer slide list stores the values from repeated calls to
Slide::getName().c_str() in a large list for filtering purposes.
However, the temporary SkString was being destroyed before the list was
complete. This luckily happened to work because SkString's refcounting
behavior kept the internal buffer alive past the lifetime of the
temporary SkString object, but it's not intended to work.

Returning a const-ref to the internal SkString is more efficient, and
gives us long-lived SkStrings to work with.

Change-Id: I958148ba46332f9ec576ac60d0a9b916284c4761
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/306256
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>

tools/viewer/Slide.h

@@ -34,7 +34,7 @@ public:
     virtual bool onGetControls(SkMetaData*) { return false; }
     virtual void onSetControls(const SkMetaData&) {}
 
-    SkString getName() { return fName; }
+    const SkString& getName() { return fName; }
 
 
 protected: