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/DisplacementBench.cpp b/bench/DisplacementBench.cpp
index b83d0d1..d9ddc8b 100644
--- a/bench/DisplacementBench.cpp
+++ b/bench/DisplacementBench.cpp
@@ -23,7 +23,7 @@
     }
 
 protected:
-    void onPreDraw() override {
+    void onDelayedSetup() override {
         if (!fInitialized) {
             this->makeBitmap();
             this->makeCheckerboard();