Spin off GM::runAsBench() from flags.

This will let us kill flags.

BUG=skia:

Review URL: https://codereview.chromium.org/873753002
diff --git a/gm/gm.h b/gm/gm.h
index 81744e2..55820ac 100644
--- a/gm/gm.h
+++ b/gm/gm.h
@@ -53,8 +53,6 @@
 
             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 @@
         SkISize getISize() { return this->onISize(); }
         const char* getName();
 
+        virtual bool runAsBench() const { return false; }
+
         uint32_t getFlags() const {
             return this->onGetFlags();
         }