Add ability to query read pixels support without a render target.

src/gpu/gl/GrGLGpu.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLGpu_DEFINED
 #define GrGLGpu_DEFINED
 
@@ skipping 148 matching lines @@
     // compatible stencil format, or negative if there is no compatible stencil format.
     int getCompatibleStencilIndex(GrPixelConfig config);
 
     void onClear(GrRenderTarget*, const SkIRect& rect, GrColor color) override;
 
     void onClearStencilClip(GrRenderTarget*, const SkIRect& rect, bool insideClip) override;
 
     bool onMakeCopyForTextureParams(GrTexture*, const GrTextureParams&,
                                     GrTextureProducer::CopyParams*) const override;
 
+    // Checks whether glReadPixels can be called to get pixel values in readConfig from the
+    // render target.
+    bool readPixelsSupported(GrRenderTarget* target, GrPixelConfig readConfig);
+
+    // Checks whether glReadPixels can be called to get pixel values in readConfig from a
+    // render target that has renderTargetConfig. This may have to create a temporary
+    // render target and thus is less preferable than the variant that takes a render target.
+    bool readPixelsSupported(GrPixelConfig renderTargetConfig, GrPixelConfig readConfig);
+
+    // Checks whether glReadPixels can be called to get pixel values in readConfig from a
+    // render target that has the same config as surfaceForConfig. Calls one of the the two
+    // variations above, depending on whether the surface is a render target or not.
+    bool readPixelsSupported(GrSurface* surfaceForConfig, GrPixelConfig readConfig);
+
     bool onReadPixels(GrSurface*,
                       int left, int top,
                       int width, int height,
                       GrPixelConfig,
                       void* buffer,
                       size_t rowBytes) override;
 
     bool onWritePixels(GrSurface*,
                        int left, int top, int width, int height,
                        GrPixelConfig config, const void* buffer,
@@ skipping 388 matching lines @@
     } fPLSSetupProgram;
 
     bool fHWPLSEnabled;
     bool fPLSHasBeenUsed;
 
     typedef GrGpu INHERITED;
     friend class GrGLPathRendering; // For accessing setTextureUnit.
 };
 
 #endif