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/DrawBitmapAABench.cpp b/bench/DrawBitmapAABench.cpp
index e4e0296..9d5003b 100644
--- a/bench/DrawBitmapAABench.cpp
+++ b/bench/DrawBitmapAABench.cpp
@@ -32,7 +32,7 @@
         return fName.c_str();
     }
 
-    void onPreDraw() override {
+    void onDelayedSetup() override {
         fBitmap.allocN32Pixels(200, 200);
         fBitmap.eraseARGB(255, 0, 255, 0);
     }