Make GrContextThreadSafeProxy not a GrContext_Base

Once this API is retracted, we can rename it to something more sane.
The code base has some `fContextInfo` ivars of this type, suggesting it
was previously named ContextInfo. It could be a ContextGroup or something else.

Bug: skia:10318
Change-Id: I3471e2172f46163f98a94780f0d7eb3431894cda
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293556
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: Adlai Holler <adlai@google.com>
diff --git a/include/private/GrImageContext.h b/include/private/GrImageContext.h
index 9dc0acb..65d3064 100644
--- a/include/private/GrImageContext.h
+++ b/include/private/GrImageContext.h
@@ -18,10 +18,6 @@
 public:
     ~GrImageContext() override;
 
-    SK_API GrBackendFormat defaultBackendFormat(SkColorType ct, GrRenderable renderable) const {
-        return INHERITED::defaultBackendFormat(ct, renderable);
-    }
-
     // Provides access to functions that aren't part of the public API.
     GrImageContextPriv priv();
     const GrImageContextPriv priv() const;
@@ -29,7 +25,7 @@
 protected:
     friend class GrImageContextPriv; // for hidden functions
 
-    GrImageContext(GrBackendApi, const GrContextOptions&, uint32_t contextID);
+    GrImageContext(sk_sp<GrContextThreadSafeProxy>);
 
     SK_API virtual void abandonContext();
     SK_API virtual bool abandoned();
@@ -44,7 +40,6 @@
 
 private:
     std::unique_ptr<GrProxyProvider> fProxyProvider;
-    bool                             fAbandoned = false;
 
     // In debug builds we guard against improper thread handling
     // This guard is passed to the GrDrawingManager and, from there to all the