Initial version of external_oes texture support and unit test

include/gpu/gl/angle/SkANGLEGLContext.h

Index: include/gpu/gl/angle/SkANGLEGLContext.h
diff --git a/include/gpu/gl/angle/SkANGLEGLContext.h b/include/gpu/gl/angle/SkANGLEGLContext.h
index 8d5f83041a9d4a268fad8432297a281605b2a469..7858fff9649de75f4296936760bab8a5a4c47dc0 100644
--- a/include/gpu/gl/angle/SkANGLEGLContext.h
+++ b/include/gpu/gl/angle/SkANGLEGLContext.h
@@ -27,6 +27,10 @@ public:
         }
         return ctx;
     }
+    GrEGLImage texture2DToEGLImage(GrGLuint texID) const override;
+    void destroyEGLImage(GrEGLImage) const override;
+    GrGLuint eglImageToExternalTexture(GrEGLImage) const override;
+    SkGLContext* createNew() const override;
 
     // The param is an EGLNativeDisplayType and the return is an EGLDispay.
     static void* GetD3DEGLDisplay(void* nativeDisplay, bool useGLBackend);
@@ -42,6 +46,7 @@ private:
     void* fContext;
     void* fDisplay;
     void* fSurface;
+    bool  fIsGLBackend;
 };
 
 #endif