Add postFlush call to GrOnFlushCallbackObject

Adds a new postFlush method and renames the class to
GrOnFlushCallbackObject. Also removes the ref counting in favor of
making the callback object a purely virtual interface. ref/unref on the
callback interface would conflict with existing ref/unref methods on the
subclass. It is now the caller’s responsibility to ensure the lifetime
of the callback is tied to that of the context.

Bug: skia:
Change-Id: I2fc1f98c700032e296a36f3a9a09c0753ab47aea
Reviewed-on: https://skia-review.googlesource.com/15463
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 2b34232..92dc7d6 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -122,8 +122,8 @@
   "$_src/gpu/GrPathUtils.cpp",
   "$_src/gpu/GrPathUtils.h",
   "$_src/gpu/GrPendingProgramElement.h",
-  "$_src/gpu/GrPreFlushResourceProvider.cpp",
-  "$_src/gpu/GrPreFlushResourceProvider.h",
+  "$_src/gpu/GrOnFlushResourceProvider.cpp",
+  "$_src/gpu/GrOnFlushResourceProvider.h",
   "$_src/gpu/GrPipeline.cpp",
   "$_src/gpu/GrPipeline.h",
   "$_src/gpu/GrPipelineBuilder.h",