Add GrBackendTexture accessor to SkImage (take 2)

This makes accessing the GPU resource behind an SkImage a lot more typesafe. Additionally, the GrBackendObject is being deprecated so this is the path forward.

I split the controversial stuff off into https://skia-review.googlesource.com/c/skia/+/118575 (Add SkImage::setLayout call).

Change-Id: I297e72770e8fb360fac7c7cd74f050ae759ae133
Reviewed-on: https://skia-review.googlesource.com/118571
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/docs/SkImage_Reference.bmh b/docs/SkImage_Reference.bmh
index 781ad71..7aad9ae 100644
--- a/docs/SkImage_Reference.bmh
+++ b/docs/SkImage_Reference.bmh
@@ -1326,7 +1326,7 @@
 Retrieves the back-end API handle of texture. If flushPendingGrContextIO is true,
 complete deferred I/O operations.
 
-If origin in not nullptr, copies location of content drawn into Image.
+If origin is not nullptr, copies location of content drawn into Image.
 
 #Param flushPendingGrContextIO  flag to flush outstanding requests ##
 #Param origin  storage for one of: kTopLeft_GrSurfaceOrigin,
@@ -1390,6 +1390,32 @@
 
 # ------------------------------------------------------------------------------
 
+#Method GrBackendTexture getBackendTexture(bool flushPendingGrContextIO,
+                                           GrSurfaceOrigin* origin = nullptr) const
+#In Property
+#Line # returns GPU reference to Image as texture ##
+
+Retrieves the backend texture. If there is none an invalid object will be returned.
+If flushPendingGrContextIO is true, complete deferred I/O operations.
+
+If origin in not nullptr, copies location of content drawn into Image.
+
+#Param flushPendingGrContextIO  flag to flush outstanding requests ##
+#Param origin  storage for one of: kTopLeft_GrSurfaceOrigin,
+               kBottomLeft_GrSurfaceOrigin; or nullptr
+##
+
+#Return back-end API texture handle. Invalid on failure. ##
+
+#NoExample
+##
+
+#SeeAlso MakeFromTexture isTextureBacked
+
+#Method ##
+
+# ------------------------------------------------------------------------------
+
 #Enum CachingHint
 
 #Code