Add postFlush call to GrOnFlushCallbackObject

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>