do work in onBefore, not in constructor

Don't want to do any work in dm if we're not going to actually run
a given test/gm

Change-Id: I16adf62729747f86de94bdd62228fbce31be0a85
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330942
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/gm/dashcubics.cpp b/gm/dashcubics.cpp
index a6abadf..3faceef 100644
--- a/gm/dashcubics.cpp
+++ b/gm/dashcubics.cpp
@@ -74,7 +74,9 @@
 
 class TrimGM : public skiagm::GM {
 public:
-    TrimGM() {
+    TrimGM() {}
+
+    void onOnceBeforeDraw() override {
         SkAssertResult(SkParsePath::FromSVGString(
             "M   0,100 C  10, 50 190, 50 200,100"
             "M 200,100 C 210,150 390,150 400,100"