Fix for nexus 5 crashing in GL benches

GLBenches do not expect gl state  to change between onPerCanvasPreDraw and *PostDraw, but we do a clear and sometimes we clear as draw.  This causes us to bind vertex objects / programs / etc.

This change creates two new virtual methods which are called right before and immediately after timing.

BUG=skia:

Review URL: https://codereview.chromium.org/1379853003
diff --git a/bench/MergeBench.cpp b/bench/MergeBench.cpp
index 0a2611d..404f646 100644
--- a/bench/MergeBench.cpp
+++ b/bench/MergeBench.cpp
@@ -25,7 +25,7 @@
         return fIsSmall ? "merge_small" : "merge_large";
     }
 
-    void onPreDraw() override {
+    void onDelayedSetup() override {
         if (!fInitialized) {
             make_bitmap();
             make_checkerboard();