add saveBehind to canvas

Bug: skia:
Change-Id: I62e48b116b2d7efef585d4f465441f24ae4d2b29
Reviewed-on: https://skia-review.googlesource.com/c/177892
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/src/gpu/SkGpuDevice.h b/src/gpu/SkGpuDevice.h
index dfc5fc7..d14572a 100644
--- a/src/gpu/SkGpuDevice.h
+++ b/src/gpu/SkGpuDevice.h
@@ -115,6 +115,7 @@
     sk_sp<SkSpecialImage> makeSpecial(const SkBitmap&) override;
     sk_sp<SkSpecialImage> makeSpecial(const SkImage*) override;
     sk_sp<SkSpecialImage> snapSpecial() override;
+    sk_sp<SkSpecialImage> snapBackImage(const SkIRect&) override;
 
     void flush() override;
     GrSemaphoresSubmitted flushAndSignalSemaphores(int numSemaphores,