Fix up GrAuditTrail to allow arbitrary reordering

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1765123002

Review URL: https://codereview.chromium.org/1765123002
diff --git a/tools/skiaserve/Request.cpp b/tools/skiaserve/Request.cpp
index 46e3039..86527f8 100644
--- a/tools/skiaserve/Request.cpp
+++ b/tools/skiaserve/Request.cpp
@@ -150,6 +150,7 @@
 
     // for some reason we need to 'flush' the debug canvas by drawing all of the ops
     fDebugCanvas->drawTo(this->getCanvas(), this->getLastOp());
+    this->getCanvas()->flush();
     return true;
 }