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/ColorPrivBench.cpp b/bench/ColorPrivBench.cpp
index 9c8acce..4aa51c4 100644
--- a/bench/ColorPrivBench.cpp
+++ b/bench/ColorPrivBench.cpp
@@ -24,7 +24,7 @@
 
     const char* onGetName() override { return fName.c_str(); }
 
-    void onPreDraw() override {
+    void onDelayedSetup() override {
         // A handful of random srcs and dsts.
         SkRandom rand;
         for (int i = 0; i < kInputs; i++) {