Make GrTexture caching for SkPromiseImageTexture work when same texture
fulfills a different SkImage.

Bug: skia:8613
Change-Id: I7ee14112c69f8aaef223a37dda455259b501a2bf
Reviewed-on: https://skia-review.googlesource.com/c/184440
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/mock/GrMockTexture.h b/src/gpu/mock/GrMockTexture.h
index ac5eab0..3674de6 100644
--- a/src/gpu/mock/GrMockTexture.h
+++ b/src/gpu/mock/GrMockTexture.h
@@ -52,6 +52,7 @@
         fIdleProc = proc;
         fIdleProcContext = context;
     }
+    void* idleContext() const override { return fIdleProcContext; }
 
 protected:
     // constructor for subclasses