disable flaky gm for now

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1689653002

TBR=

Review URL: https://codereview.chromium.org/1689653002

gm/xferu64.cpp

@@ -96,6 +96,8 @@ static void draw_rect(SkCanvas* canvas, const SkRect& r, SkColor c, uint32_t u64
  *  Test SkXfer4fProcs directly for src-over, comparing them to current SkColor blits.
  */
 DEF_SIMPLE_GM(xfer_u64_srcover, canvas, 580, 760) {
+    return; // disable for now, until we understand the failures on Nexus 5 etc.
+
     const int IW = 50;
     const SkScalar W = IW;
     const SkScalar H = 100;