Spin off GM::runAsBench() from flags.

gm/gm.h

Index: gm/gm.h
diff --git a/gm/gm.h b/gm/gm.h
index 81744e2512625261c029d2fb68e926cc5cebdbad..55820ac61c2d9fe05af4bd0f99bd6e0179e91fc4 100644
--- a/gm/gm.h
+++ b/gm/gm.h
@@ -53,8 +53,6 @@ namespace skiagm {
 
             kGPUOnly_Flag               = 1 << 9,
 
-            kAsBench_Flag               = 1 << 10, // Run the GM as a benchmark in the bench tool
-
             kNoBBH_Flag                 = 1 << 11, // May draw wrong using a bounding-box hierarchy
         };
 
@@ -74,6 +72,8 @@ namespace skiagm {
         SkISize getISize() { return this->onISize(); }
         const char* getName();
 
+        virtual bool runAsBench() const { return false; }
+
         uint32_t getFlags() const {
             return this->onGetFlags();
         }