Expose ManagedBackendTexture from BackendTextureImageFactory.

Add helper to create self-managed BackendTexture-backed SkSurface for
tests using MBET.

GrGpu::createTestingOnlyBackendRenderTarget supports protected.

Make SkSurfaceCharacterization tests use self-managed SkSurface
factories and a use case of MakeFromBackendTextureAsRenderTarget is
removed.

Use self-managed BackendTexture-backed SkSurface factory in DM sinks and
in fm.

Bug: skia:9832

Change-Id: I0c1dc49697f8b3c942864e18b9112a3552f431ba
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/323559
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 2006516..66b957d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1606,6 +1606,8 @@
       "tools/gpu/FlushFinishTracker.h",
       "tools/gpu/GrContextFactory.cpp",
       "tools/gpu/GrTest.cpp",
+      "tools/gpu/ManagedBackendTexture.cpp",
+      "tools/gpu/ManagedBackendTexture.h",
       "tools/gpu/MemoryCache.cpp",
       "tools/gpu/MemoryCache.h",
       "tools/gpu/ProxyUtils.cpp",