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/MipMapBench.cpp b/bench/MipMapBench.cpp
index 18af1e5..2fb52bb 100644
--- a/bench/MipMapBench.cpp
+++ b/bench/MipMapBench.cpp
@@ -22,7 +22,7 @@
 
     const char* onGetName() override { return "mipmap_build"; }
 
-    void onPreDraw() override {
+    void onDelayedSetup() override {
         fBitmap.allocN32Pixels(1000, 1000, true);
         fBitmap.eraseColor(SK_ColorWHITE);  // so we don't read uninitialized memory
     }