Add GrPrepareCallback, always run at the start of flush

This is like an op, but only has one virtual, and always runs before
any ops prepare. To be used in threaded software mask rendering
(to schedule ASAP uploads).

Bug: skia:
Change-Id: I647482e2472d7321f3685e5bdbe49e10ac59c0b1
Reviewed-on: https://skia-review.googlesource.com/37160
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/GrRenderTargetOpList.h b/src/gpu/GrRenderTargetOpList.h
index f162fcc..b62cee5 100644
--- a/src/gpu/GrRenderTargetOpList.h
+++ b/src/gpu/GrRenderTargetOpList.h
@@ -65,8 +65,8 @@
      * Together these two functions flush all queued up draws to GrCommandBuffer. The return value
      * of executeOps() indicates whether any commands were actually issued to the GPU.
      */
-    void prepareOps(GrOpFlushState* flushState) override;
-    bool executeOps(GrOpFlushState* flushState) override;
+    void onPrepare(GrOpFlushState* flushState) override;
+    bool onExecute(GrOpFlushState* flushState) override;
 
     uint32_t addOp(std::unique_ptr<GrOp> op, const GrCaps& caps) {
         this->recordOp(std::move(op), caps, nullptr, nullptr);